GB2065919A

GB2065919A - Focusing screen

Info

Publication number: GB2065919A
Application number: GB8035647A
Authority: GB
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1979-11-07
Filing date: 1980-11-06
Publication date: 1981-07-01
Also published as: DE3041969C2; DE3041969A1; GB2065919B

Abstract

This specification discloses a split image type focusing screen. This screen comprises a pair of portions 2, 3 disposed in the center of the real image plane of the camera, each portion having a refracting means and a diffracting means. In the embodiments described each portion has a prism with a diffraction lattice thereon Fig. 10A. The prisms are oriented in opposite directions so that one portion deflects the incident light mainly in one direction and the other portion deflects it mainly in a second different direction in a defocused condition the images viewed through the portions are split, blurred, and may also be coloured e.g. blue, red, due to the diffraction, Fig. 10B. This screen can be used with an objective lens having a small aperture ratio. <IMAGE>

Description

SPECIFICATION Focusing screen BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a focusing screen adapted to be mounted in the optical viewing path of a camera.

Description of the Prior Art Cameras of the single lens reflex type and also of the range finder type generally include a focus indicating system utilizing an image splitting bi-prism. This focus indicating system usually comprises a focusing screen having a focus indicating part disposed in the center of the real image plane of the range finder and comprising image splitting prisms.

The focus indicating system of such split-image type, as compared with other systems, has a good focusing accuracy, but suffers from a phenomenon that the focus indicating part dark-changes for an interchangeable lens having a great F-number, namely, low in brightness.

As the aperture ratio of the lens is decreased, the image splitting bi-prism become larger dark areas. Accordingly, the focusing becomes more difficult with increasing F-number of the lens.

This focus indicating accuracy and the dark of the focus indicating part have a relation contrary to each other (inversely proportional to each other) for the vertical angle of the image splitting prism. This contrary relation is attributable to the fact that, among the light beams from the exit pupil of the lens, only the light beam having a particular angle of incidence determined by the vertical angle of the image splitting prism passes through the range finder and reaches the eye of the observer and contributes to the focus indicating. That is, if the vertical angle of the splitting prism is increased, the light rays from the marginal portion of the exit pupil of the phototaking lens can be directed to the eye-piece of the view finder and thus, the focus indicating accuracy is enhanced.However, a splitting prism having a great vertical angle has its focus indicating part dark-changeable even for an interchangeable lens which is a little dark, namely, which has a great F-number, and thus, the focus indicating becomes impossible.

Conversely, if the vertical angle of the prism is made small, the focus indicating accuracy is reduced but the focus indicating is possible even for an interchangeable lens which has a great Fnumber, namely, which is considerably dark.

To overcome such phenomenon, the vertical angle of the image splitting prism usually is not rendered to so great a value but is minimized to an angle in the vicinity of 80 so that, even if the focus indicating accuracy is sacrificed to some extent, the focus indicating becomes possible even for a dark interchangeable lens having an F-number of the order of 5.6.

However, for interchangeable lenses of a smaller maximum aperture ratio, for example, f/8 and larger, the aforementioned problem of dark prism is again presented.

An object of the present invention is to provide a focusing screen so as at least partially to alleviate the above problems.

One aspect of the invention provides a focusing screen which enables a focus condition to be easily detected.

Another aspect of the present invention provides a focusing screen which has a small degree of dark and can be used even if a dark lens having a great F-number is used.

A further aspect of the present invention provides a focusing screen in which the relation between the aforementioned focus indicating accuracy and the dark of the prism is alleviated and even if 3 dark interchangeable lens is used, the focus indicating is possible at high focusing accuracy.

The invention will become fully apparent from the following detailed description thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a focusing screen.

Figure 2 is a cross-sectional view of the light deflecting portion 2 of the focusing screen.

Figure 3 shows the functional resolution of the light deflecting portion 2.

Figure 4 shows the diffraction efficiency of the light deflecting portion.

Figures 5A and 5B illustrate the action which is imparted to an image by the light deflecting portion.

Figure 6 illustraters the manner in which the incident light bean from the exit pupil of a lens is deflected by the light deflecting portion.

Figure 7 is a cross-sectional view of the light deflecting portion according to another embodiment.

Figure 8 is a cross-sectional view of a three-step type diffraction lattice.

Figure 9 illustrates the diffraction efficiency of the three-step diffraction lattice.

Figure 1 OA is a cross-sectional view of a light deflecting portion in which the three-step type diffraction lattice is provided on a prism.

Figure lOB shows a linear image viewed through a focus indicating part using the light deflecting portion of Figure 1 OA.

Figure 11 shows a modification of the light deflecting portion of Figure 1 OA.

Figure 12 illustrates the diffraction efficiency of a light deflecting portion in which a four-step type diffraction lattice is provided on a prism Figure 13 is a schematic view of the light deflecting portion according to still another embodiment.

Figure 14 is a cross-sectional view of the light deflecting portion of Figure 13.

Figure 1 5 shows the functional resolution of the light deflecting portion of Figure 14.

Figure 1 6 is a schematic view of a focus indicating part comprising a combination of two light deflecting portions as shown in Figure 1 3.

Figures 1 7 and 1 8 are cross-sectional views of the light deflecting portions according to different embodiments.

Figures 1 9A and 1 9B are a perspective and a cross-sectional view, respectively, of a focus indicating part comprising a diffraction lattice alone.

Figure 20 is a graph of the diffraction term of the diffraction lattice shown in figure 1 9B.

Figure 21 is a graph of the diffracted light distribution of the Figure 1 9B diffraction lattice for white light.

Figures 22A, 22B and 22C are a plan view of the focusing screen according to another embodiment of the present invention, a perspective view of the focus indicating part and a crosssectional view of the light deflecting portion, respectively.

Figure 23 is a graph of the diffraction term of the light deflecting portion shown in Figure 22C.

Figure 24 is a graph of the contribution for diffraction term of minute prism portions 53, 54.

Figures 25, 26, 27 and 28 shows the diffracted light distributions of the light deflecting portion of Figure 22C in numerical value design examples 1.2, 3 and 4.

Figure 29 shows the dark of the focus indicating part using the light deflecting portions of Figures 16,17 and 18.

Figure 30 shows the correlation between the synchronization of the diffraction lattice and the angle of the incidence of the incident light utilizable for the focus indicating.

Figures 31, 32 and 33 show the diffracted light distributions in design examples, 5, 6 and 7.

Figure 34 is a cross-sectional view of the light deflecting portion according to still another embodiment.

Figure 35 illustrates a method of making a metal mold for mass-producing the light deflecting portion shown in Figures 22B and 22C.

Figures 36A and 36B show the focusing screen in a first application.

Figures 37A,37B,37C and 37D show the focusing screen in a second application.

Figures 38A and 38B show the focusing screen in a third application.

Figure 39a shows the focusing screen in a fourth application.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, it shows the whole of a focusing screen which is a first embodiment of the present invention. In Figure 1, reference numeral 1 designates the focusing screen which is supported in the range finder of a single lens reflex camera coincidentally with the image plane thereof. Designated by 2 and 3 are a pair of light deflecting portions for split image which together form a focus indicating part which is mounted in the center of the focusing screen. An enlarged cross-sectional view of the light deflecting portion 2 is shown in Figure 2.The light deflecting portion 2 for split image shown in Figure 2 comprises a prism 4 having a vertical angle çS as shown in Figure 3 and a phase type rectangular diffraction lattice 5 having concavo-convexity of a width of 1/2 of period P, said prism 4 and said diffraction lattice 5 being integrally overlapped with each other. That is, this light deflecting portion is of such a construction that a concave-convex phase type rectangular diffraction grating is provided on a prism having a vertical angle 0.

Accordingly, a light ray 6 which has entered the light deflecting portion 2 from an objective lens 8 is deflected by S by the refracting action of the prism 4 as shown in the functional resolution diagram of Figure 3, and is further diffracted by the phase diffraction to provide a collection 7 of multiple orders of diffracted lights.

On the other hand, the light deflecting portion 3, like the portion 2, comprises a prism and a diffractrion lattice overiapped with each other, but since the prism and the diffraction lattice are oppositely oriented, a light ray entering the portion 3 is deflected by --6 and diffracted.

As is well-known, where the refractive index of the lattice member is n and the amount of concavo-convexity is d, the n-order diffraction efficiency (= diffraction degree light intensity/incident light intensity lin of this concavo-convex phase diffraction lattice for a certain wavelength A of visible light is:

n=0 n odd number n even number where a =(n - 1 )d/. Figure 4 shows the graph of rln when a was varied by varying the amount of concavo-convexity d. Accordingly, in the case of the diffraction lattice of this type, there are chiefly produced O-order and + 1St order diffracted lights.Also, the angle 0 between the 1 st order diffracted light and the O-order diffracted light is 0 = sin (;t/p).

Here, when the diffraction lattice conditions in the present embodiment are set so that n=1.5, A= 0.5cm, d=2A=1.0,um, namely, a=x, then 'io=0 and w7+1=40%, as is apparent from Figure 4, and substantially only * 1sot order diffracted light is produced.Also, when the period p=28.6,um, the angle between the + 1st order diffracted light and the O-order diffracted light is 0=1 and accordingly, if the vertical angle 0 (shown in Figure 3) of the prism is secured so that 6=50, the light beam having entered from the plane side produces only *1sot order diffracted lights if high order diffracted lights are neglected, and they exit in the directions of 40 and 60, respectively, with respect to the optical axis.By this action, the object image, when in its defocused condition, is laterally deviated and separated by the split image prism 4 shown in Figure 3 and further, by the action of the diffraction lattice 5, each laterally deviated and separated image itself is separated into a double image and, when this double image separation is small, it is viewed as a blur of the image. This phenomenon will be fully described by reference to Figure 5. The direction of incidence of light ray is reverse to that in Figure 3, but the action of the light deflecting portion is the same as that in Figure 3. The light deflecting portion 2 shown in Figure 2 does not produce any diffracted wave when in its focus condition, and the image may be viewed without any inconvenience. However, when in its defocus (out of focus) condition, the light deflecting portion 2 produces diffracted waves as shown in Figure 5A.That is, a light beam 11 entering the focus indicating part so as to be converged at a point 12 on the optical axis is refracted in the direction of an angle S=50 by the prism action of the prism-like structure 2 and also is diffracted by the action of the diffraction lattice in the directions of +1 O on the opposite sides thereof, and point images 1 3 and 14 are formed by the respective light rays. Accordingly, when this condition is viewed through a finder, as shown in Figure 5B, the object image (shown as linear images in the Figure) whose upper and lower portions are laterally deviated from each other by the split image action of the light deflecting portions 2 and 3 will be observed and the object image will be observed as being separated into a double image.Designated by 13' and 14' are image points separated by the light deflecting portion 3. If the degree of separation of the image is made small by the diffraction lattice, the object image will be observed as being blurred rather than as a double image. Further, although slightly, the marginal portion of the image is colored by dispersion of the diffraction lattice.

As regards the linear image in defocus condition thus observed on this focus indicating part, when the double image by the + 1st order diffracted light is indicated on the upper and lower light deflecting portions as shown in figure 5B and the focus condition has been brought about, that is, the obiect image has become accurately coincident with the focusing screen, the blur of the image (or double image) by the diffraction disappears and the upper and lower images separated with the boundary between the positions 2 and 3 as the border become coincident with each other. By this phenomenon, the focus condition can be easily detected.

Incidentally, as shown in Figure 6, in the present embodiment, the light deflecting portion 2 having the aforementioned diffraction condition in the focus indicating part effectively directs chiefly to an eyepiece (not shown)) the light rays 9 and 10 of the light beam from the exit pupil 8 of the phototaking lens which form angles of 40 and 60, respectively, with respect to the optical axis. The other deflecting portion 3 directs to the eye-piece the light rays which enter from the opposite side to the light rays 9 and 10 with respect to the optical axis.

In this manner, each portion of the focus indicating part of the present invention utilizes for the focus indicating the two light rays 9 and 10 different in angle of incidence coming from the interchangeable lens and therefore, the range of F-number of the lens for which the focus indicating is possible becomes wider.

That is, in an ordinary prism type focus indicating system wherein the vertical angle of the prism is equal to that of the light deflecting portions 2, 3 of the present embodiment, a light ray 35 (Figure 6) forming an angle of 5 with respect to the optical axis is directed chiefly to the eye-piece and utilized for the focus indicating and therefore, in a lens having a great F-number wherein the light ray 35 is kicked by the eye-piece after being deflected by the prism, the focus indicating is difficult, whereas in the focus indicating part of the present embodiment, the two light rays 9 and 10 different in angle of incidence coming from the lens exit pupil are utilized for the focus indicating and therefore, even in a case where the light ray 10 does not enter a dark lens having a great F-number, the focus indicating is possible if the light ray 9 enters the lens. If the aforementioned numerical value is employed, the focus indicating part does not dark-change even for a lens having such a small exit pupil that there is only present the light ray 9 forming an angle of maximum 40 with respect to the optical axis, and thus the focus indicating is possible.

Thus, the focus indicating part of the present embodiment can accomplish the focus indicating of higher accuracy that when both the light rays 9 and 10 are used, for a bright lens having a small Fnumber, and can accomplish the focus indicating by using the light ray 9, for a dark lens having a great F-number.

Further, by varying the pitch of the diffraction lattice and the vertical angle of the split prism, utilization of a light ray forming what degree of angle with respect to the optical axis for the purpose of focus indicating can be controlled in designing. Accordingly, an optimal focus indicating part can be designed in accordance with each camera system.

Figure 7 shows a focusing screan in which image splitting prisms have been replaced with Fresnel prisms. In this focusing screen, a set of concavo-convex structures of a diffraction lattice 31 are mounted on each Fresnel band of a light deflecting portion 33. The vertical angle of each Fresnel band is equal to the vertical angle 0 of the former prism.

In the foregoing descripition, the O-order diffraction efficiency has been zero, whereas the focusing screen of the present invention is not restricted thereto. For example, if the lattice is designed such that the O-order and the + 1st order diffraction efficiency are equal, a triple image instead of a double image will appear on each of the upper and lower portions in Figure 5B, but this is not an obstruction in the present invention. Rather, each splitting image will look further blurred and in some cases, this will be effective.

When an ordinary rectangular diffraction lattice as indicated at 5 in Figure 3 is used, the *1sot and the st order diffraction efficiency become equal to arbitrary a, as shown in Figure 4. This shows that when d is constant, x71(A)+77~1(A) for an arbitrary wavelength A. Accordingly, the color of the + 1 at order diffracted light beam is the same as the color of the st order diffracted light beam, and ths images 13 and 14 in Figure 5B looks substantially the same color and only in the marginal portion of the images, there is produced the coloring resulting from the dispersion of the diffraction lattice.As an embodiment for causing such coloring to be produced more apparently, description will now be made of a case where a special rectangular diffraction lattice as shown below is used.

As shown in the article "Color Separation Grating" appearing in Applied Optics 17(15), 2273, 1978, a rectangular diffraction lattice having a staircase-like structure as shown in Figure 8 has a color filter-like action. In Figure 8, the width of each step is 1/3 of the pitch P. As shown in said article, by the diffraction lattice conditions, the 1 st order, the O-order and the st order diffracted light beam can be colored to three primary colors, i.e., red, green and blue. Also, if a diffraction lattice configuration is designed by utilizing the equations 8 and 9 appearing in page 2274 of the same article, there may be obtained a diffraction lattice having a diffracting action which divides an incident white light into two light beams different in color. An example of this is shown in Figure 9.

Figure 9 is a graph showing wavelength dependency of the diffraction efficiency when the refractive index of the diffraction lattice member is 1.5 and the step differences are d, = 1 .67,um and d2 = 3.34yam. As can be seen from this graph, the blue light of the white incident light is divided into the 1 st order diffracted light beam, the red light of the white incident light is divided into the st order diffracted light beam and the O-order diffracted light beam can be very much weakened in the visible range.

Accordingly, in a focus indicating part in which a light deflecting portion as shown in Figure 1 OA wherein such a diffraction lattice is provided on an image splitting prism is combined with an oppositely oriented portion, the defocused white linear image is such that a double image separated chiefly into red and blue as shown in Figure lOB is separated into upper and lower portions and halves of the image have relative displacement.

In Figure 1 OB, reference numerals 1 5 and 1 7 designate blue images formed by the 1 st order diffracted light, and reference numerals 1 6 and 1 8 denote red images formed by the -1st order diffracted light. The O-order diffracted light has been neglected because it is weak in intensity. At the area in which the images 1 5 and 16 overlap each other, there is seen a white image to which red and blue are added, and a blue and a red image are seen around said area, and further, these are laterally deviated in the upper and lower portions.

When the real image has become coincident with the proper plane of the light deflecting portion, the blue image 1 5 and the red image 1 6 overlap each other and likewise, the blue image 1 7 and the red image 1 8 overlap each other, whereby the coloring of red and blue disappears and the right color of the object can be observed. Any relative displacement of the two upper and lower halves of the image is not observed.

If d1 and d2 in Figure 8 are replaced with each other, the +1st order and the st order diffracted light in Figure 9 are replaced with each other. Accordingly, if, for example, the values of d1 and d2 are reversed in the upper semicircular light deflecting portion and the lower semicircular light deflecting portion of the focus indicating part, the coloring in the lower portion will be created in the direction opposite to the direction in which the coloring in the upper portion appears.

The widths of the concave and convex portions need not always be equal to each other. An embodiment using a Fresnel-like prism as a prism on which the coloring type diffraction lattice shown in Figure 10 is provided is shown in Figure 11.

Further, Figure 1 2 shows an example of the calculation of the diffraction efficiency in a light deflecting portion wherein a 4-step rectangular diffraction lattice having one more step added to the 3step rectangular diffraction lattice shown in Figure 8 is provided on a prism. The concave-convexity step differences d,, d2 and d3 are 0.96 ,um, 1.92 ,um and 2.88 cm, respectively. In the focus indicating part of this embodiment, the O-order light beam and the 1 sot order light beam are colored into blue and red, respectively. The -1st order diffracted light beam is very weak in the range of visible light.Also, in this case, unlike the case of Figure 9, the image formed by the 1 st order diffracted light is utilized and therefore, where the period of the diffraction lattice is the same as that of Figure 9, there is produced a more proximate double image. Thus, it is generally possible to utilize an n-step rectangular diffraction lattice.

The foregoing description has been made with respect chiefly to the case where a rectangular step difference diffraction lattice is provided on a prism, whereas the diffraction structure provided on the prism is not restricted thereto. For example, utilization may be made of a diffraction lattice having a sinusoidal wave-like concave-convex structure, as well as a diffraction lattice having a relief structure of the saw-tooth wave type or the roof type. Also, a diffraction lattice comprising a minute prism-like structure as shown hereinafter may be utilized.

Figure 1 3 is a perspective view of one of the light deflecting portions according to another embodiment of the present invention. Figure 14, like Figure 2, is an enlarged cross-sectional view of one of the light deflecting portions 1 9 in the split image type focus indicating part of the present embodiment. As shown in Figure i 4, this light deflecting portion is one in which, on a prism having a vertical angle 02, cuts of a minute prism-like diffraction lattice are periodically provided and portions 22 indicated by hatching have been removed. Accordingly, this light deflecting portion is functionally the same as a refraction lattice having fine cuts on a prism 23 as shown in Figure 15, and has both functions.

As can be seen from the structure of Figure 13, if viewed from another angle, this light deflecting portion comprises a periodical arrangement of minute prisms 20 having a vertical angle #2. Due to such a construction, the angle at which a light ray having entered this focus indicating part is deflected in the portion comprises a periodical arrangement of minute prisms 20 having a vertical angle 0, and minute prisms 21 having a vertical angle 02. Due to such a construction, the angle at which a light ray having entered this focus indicating part is deflected in the portion of the minute prism 20 differs from the angle at which said light ray is deflected in the portion of the minute prism 21.Accordingly, as in Figure 5, light rays different in angle coming from the entrance pupil are directed to the eye-piece.

Therefore, the focus indicating performance of this focus indicating part is also similar to that of the focus indicating part having the construction as shown in Figure 5.

Accordingly, by combining the light deflecting portion of figure 13 with the light deflecting portion 25 of Figurt: 1 6 which is similar in form thereto but oppositely oriented, there may be realized a split image type focus indicating part which will provide lateral deviation of the upper and lower images as well as blur (double image) or coloring of image.

By replacing the image splitting basic prism with a Fresnel prism as previously described in each light cFsflecting portion 1 9, 25 shown in Figure 16, it is possible to realise a light deflecting portion having the cross-section as shown in Figures 1 7 and 1 8. The light deflecting portion of such configuration has the same function as that of the light deflecting portion shown in Figure 13. In Figures 17 and 18, the light deflecting portion 35 has a unit structure of diffraction lattice formed on one Fresnel band of the basic prism, said unit structure having the same pitch but being smaller than the width of the Fresnel band.That is, a set of minute prisms 28, 29 having vertical angles sj1 and 02 correspond to one Fresnel band 30. In Figures 1 7 and 18, P designates the width of one Fresnel band, and P1 or P2 designates the width of the unit structure of the diffraction lattice.

On the other hand, if viewed from still another angle, the light deflecting portion shown in Figures 17 and 1 8 may be said to comprise a periodical arrangement of prism-like unit structures (corresponding to 30) formed by two inclined surfaces differing in angle of inclination. The use of a Fresnel prism as the image splitting basic prism leads to a smaller thickness of the focus indicating part and also leads to an advantage which will later be described.

In tl;e foregoing description of the embodiments, the effect which the diffracting action by the diffraction lattice component provided on the light deflecting portion of the focus indicating part brings about during the detection of the focus condition has been particularly explained and now, another effect which the diffracting action existing with the light deflecting action of the prism brings about will be fully described.

As previously described, the focus indicating part of the prior art using an image splitting bi-prism usually directs to the eye the light ray forming an angle of S - 40 with respect to the optical axis (the light ray 9 in Figure 6) and utilizes it for the focus indicating and therefore, for the light beam from a lens having a great F-number, for example, f/5.6 or greater in which such light ray is kicked, dark change occurs and the focus indicating cannot be accomplished. As a method which will overcome such disadvantage, a focusing screen would occur to mind in which phase type diffraction lattices of sawtooth-like cross-section as shown in Figures 1 9A and 19B are oppositely oriented in the upper and lower semicircular portions of the circular focus indicating part.However, as shown below, the aforementioned disadvantage cannot fully be eliminated by a system including such a focus indicating part formed only by ordinary diffraction lattices.

In order to know how the light beam from the exit pupil of the lens is utilized for the focus indicating in the focus indicating part shown in Figure 1 9A, a light beam is caused to enter from the direction of the optical axis of the view finder and how the diffracted light produced by this focus indicating part is distributed will be calculated. For the comparison with an embodiment of the present invention which will later be described, the distribution of the diffracted light will be calculated with the condition of this diffraction lattice set to a value approximate to that of the embodiment of the present invention.As shown in the cross-sectional view of Figure 19B, the pitch of this diffraction lattice is P = 25 ym, the angle of inclination of each wedge member is 4X = 80, the refractive index of the constituent member 40 of the focus indicating part is n = 1.49, and the wavelength of the incident light beam 42 is A = 0.55 ym. The diffracted light distribution by such a diffraction lattice, as described in Principles of Optics, written by Born and Wolf, pp.401-405, can be obtained from the product of a term providing the extent of the diffraction pattern by the unit structure of the diffraction lattice (hereinafter referred to as the diffraction term) and a diffraction spectrum determined by the lattice period P (hereinafter referred to as the interference term).Assuming that the number of the diffraction lattice lines is sufficiently great, this interference term, as is well-known, becomes the sum of - functions having a peak only at the angle of ON determined by P-sin0N = N-R (N = O, + 1, +2, . . . ). The diffracted light distribution thus obtained in the case of the aforementioned lattice condition is shown in Figure 20. As shown there, by the diffraction in this diffraction lattice, the 3rd order diffracted light is intensely created in a direction having an inclination of 40 with respect to the optical axis, and diffracted lights of the other orders are scarcely created.

Where the incident light 42 is white light, the diffracted light distribution for the white light may be obtained by sampling the wavelength A = 0.4 - 0.7 ym at intervals of 0.01 ,um, obtaining the diffracted light distribution at each wavelength, and-summing up the diffracted light distributions with the weight of the relative luminosity factor of the naked eye added thereto. This is shown in Figure 21.

The reason why the weight of the relative luminosity factor of the marked eye is added is that in a camera, the brightness of an image sensed by the naked eye when the image is viewed by the naked eye through the view finder is important.

As seen from Figure 21, the extent of the diffracted light in the case of white incident light is such that the diffraction angle 0 concentrates in the vicinity of 0 = 30--50. Accordingly, in this focus indicating part, of the light beams entering the focus indicating part from the exit pupils of the lens, only the light rays entering at an angle of 30--50 with respect to the optical axis of the view finder are directed to the observing eye so as to be utilized for the focus indicating.In the focus indicating part using the diffraction lattice shown in Figures isA and B, the range of angle of the light beam available for the focus indicating is slightly wider than in the focus indicating part using the image splitting biprism and therefore, even for interchangeable lenses having a wider range of F-number, the image on the focus indicating part does not dark-change. However, this extent is still small and insufficient.

In an embodiment which will now be described, thc range of incidence angle of the light ray available for the focus indicating is wider than that shown in Figures 1 9A and B. Each light deflecting portion in the focus indicating part of the focusing screen of the present invention has a prism zction and a diffracting action and therefore, by selecting the pitch of the diffraction lattice and the vertical angle of the prism, utilization of a light ray forming what degree of angle with respect to the optical axis of the view finder for the purpose of focus indicating can be controlled in designing. Accordingly, optimal focusing screens can be designed in accordance with various camera systems.

Some examples of the design of the focusing screen of the present invention will be shown below, and these design examples have been made for the embodiment as shown in Figures 22A, B and C.

Figures 22A is a plan view of the focusing screen 52. As shown there, a focus indicating part comprising light deflecting portions 50 and 51 is provided in the center of this screen. Figure 228 is an enlarged perspective view of the focus indicating part, and Figure 22A is a cross-sectional view of the light deflecting portion 50. As is apparent, this portion is equivalent to the light deflecting portion shown .n Figure 17, and as shown, on each Fresnel band of a Fresnel prism, a unit concave-convex structure of the diffraction lattice smaller than the width of the Fresnel band is mounted to form two light deflecting portions 50 and 51 which are oppositely combined to constitute a focus indicating part.

Each Fresnel band shown in Figure 22C is of a double wedge type structure which comprises minute prism portions 53 having a vertical angle 0, and minute prism portions 54 having a vertical angle 02. In the design examples which will hereinafter be described, the symbols of parameters are those shown in Figure 22C. That is, P,, (b, and P2, 42 are the widths and vertical angles, respectively, of the minute prism portions 53 and 54, and P is the width (or pitch) of a Fresnel band. The material forming each light deflecting portion 50, 51 is acryl having a refractive index 1.49.Further, in each design example, in order that the characteristic thereof may be compared with the example shown in Figures 1 9A and B, in what light quantity distribution the light beam 57 entering a light deflecting portion 50 from the direction of the optical axis as shown in Figure 22C is deflected with respect to the angle 0 from the optical axis will be considered hereinafter.

The angle distribution of the transmitted light by these light deflecting portions is attributable to the prism action and the diffracting action of the light diffracting portions, but in the following description of the design examples, to help comparison with the example shown in Figures 1 9A and B, the light deflecting portion shown in Figure 22C will be regarded as a kind of diffraction lattice and the aforementioned angle distribution will be explained as being attributable to the diffracted light distribution thereof.

Design Example 1: P=24ym,P,=P2= 12m,= = 10.83 ,fi2=5.66 In this Design Example 1, for comparison with Figure 20, the calculation of the diffracted light distribution for the incident light of A = 0.55 4m from the direction of the optical axis is shown in Figure 23.As compared with figure 20, the diffraction term is much widened, so that a number of each order diffracted lights are created between 0 = 1 0--70. If, as previously described, a Fresnel band of the light deflecting portion shown in Figure 22C is resolved into a minute prism portion 53 having an inclined surface 55 having an angle of inclination sJl and a minute prism portion 54 having an inclines surface 56 having an angle of inclination 02, then the diffraction term of Figure 22 will be considered to be the result of the interference between the extent 60 by the diffraction in the minute prism portion 53 and the extent 61 by the diffraction in the minute prism portion 54, as shown in Figure 24.

Further, when the diffracted light distribution for white incident light is calculated as already described with respect to Figure 21, the result will be such as shown in Figure 25. In this design example, the diffracted light distribution extends over a much wider range than in the case of Figure 21.

Accordingly, the focus indicating part of the present design example can direct a light beam of a wide incidence angle range, of the light beams from the exit pupil of the lens, to the observing eye and utilize it for the focus indicating. In the case of the focus indicating part having the characteristic shown in Figure 21, the light beam the exit pupil of a lens having a great F-number in which an incident light beam having an incidence angle of 30 or greater does not exit does not reach the observing eye and accordingly, the image on the focus indicating part dark-changes to make the focus indicating difficult.

In the focus indicating part of the present design example, even for a lens having such a great F-number, a light beam having an incidence angle of 10--30 can be directed to the observing eye and utilized for the focus indicating.

Design Example 2: P=24ym,P, =P2= 12ym,0, =8.5 ,02=5.67 The diffracted light distribution for white light entering the light deflecting portion in the focus indicating part of this Design Example 2 is shown in Figure 26. As compared with Design Example 1 (Figure 25), the present design example is characterized in that the diffracted light whose diffraction angle O is 60 or greater is reduced in quantity. Accordingly, for a lens having such an F-number that the incident light having an incidence angle of 60 or greater is small in quantity, the image on the focus indicating part is not darkened but can be used.

Design Example 3: P=24ym,Pt =P2= 12ym,)1 = 12.0 , 02= 3 5 The diffracted light distribution for white light entering the light deflecting portion in the focus indicating part of this Design Example 3 is shown in Figure 27, as compared with Design Example 1 (Figure 25) and Design Example 2 (Figure 26), this design example is characterized in that the quantity of the diffracted light between the diffraction angle 0 00--20 is great. Therefore, even for a lens having a considerably small aperture ratio, the image on the focus indicating part is little darkened.The O-order diffracted light is about 4% of the whole quantity of diffracted light and consequently, the quantity of light available for the focus indicating is great and thus, the accuracy of the focus indicating is little reduced.

Design Example 4: P = 30 Mm, P, = P2 = 15,um, s6, = 11.0 ,02 = 3-5 The diffracted light distribution for white light entering the light deflecting portion in the focus indicating part of this Design Example 4 is shown in Figure 28. In this design example, as compared with the previous three Design Examples 1, 2 and 3, the widths P1 and P2 of the minute prism portions are different. The diffracted light distribution in this design example is substantially the same as that in Design Example 3, but the quantity of O-order diffracted light is smaller than that in Design Example 3.

Four preferred design examples have been described above, and desirable conditions for further improved characteristic of the focus indicating part used in the focusing screen of the present invention will hereinafter be described.

When the focusing screen 52 of the present embodiment shown in Figures 22A, 22B and 22C is disposed in the view finder of a single lens reflex camera and the aperture of the objective lens is gradually stopped down, the focus indicating part begins to become dark from a certain F-number or greater the pattern as shown in Figure 29. The dark portion 64 in Figure 29 is an area corresponding to the minute prism portion 55 of Figures 22B and 22C.This is because this minute prism portion having an inclined surface having a great angle of inclination sustains the contribution 60 of a portion having a great diffraction angle as shown in Figure 24 for the diffraction term and, as the aperture of the objective lens is stopped down, the incident light beam corresponding to this great angle is kicked by the aperture and does not enter the focus indicating part. Accordingly, this minute prism portion having an inclined surface having a great angle of inclination dark-changes first.

It is not preferable that the image on the focus indicating part looks in the light-and-dark figure as shown in Figure 29. This may be improved by reducing the pitch of the diffraction lattice to thereby reduce the width of the minute prism portion and reducing the periodical pattern of this light-and-dark figure to such a degree of fineness that it cannot be resolved by the naked eye when the focus indicating part is viewed through the view finder. If the resolution of the naked eye is taken into account, the pitch of the diffraction lattice provided on the light deflecting portions of the focus indicating part should desirably be 50 ym or less, including the case where the focusing screen of the present invention is applied as the focusing screen of an ordinary single lens reflex camera.Accordingly, in the case of the focus indicating part of the form shown in Figures 22A, 22B and 22C, the pitch P of the prism-like unit structure (corresponding to a Fresnel band) comprising minute prism portions 53 and 54 should also desirably be 50 ym or less.

On the other hand, it is not preferable that the pitch of the diffraction lattice provided on the focus indicating part is too small. The reason will be shown below. Among the diffracted lights available for the focus indicating, it is the 1 st order diffracted light that has the-smallest diffraction angle. Because the Order diffracted light does not contribute to the image splitting even in defocus condition.

Accordingly, for a lens having such an F-number that a light beam having an incidence angle corresponding to the diffraction angle of the 1 st order diffracted light is kicked, the focus indicating part does not cause the image splitting to take place. For example, the focus indicating part of Design Example 2 having the diffracted light distribution of Figure 23 enables the focus indicating in a camera provided with a lens in which a light beam entering the focus indicating part at an incidence angle of 1.30 or greater exists, but where a lens having a smaller aperture is used, the focus indicating is impossible.

Generally, a relation that sin 0, = ;1jP (.A: wavelength of light) lies between the diffraction angle 0, of the 1 st order diffracted light and the pitch P of the diffraction lattice. When A = 0.55 ym, this relation is shown in the graph of Figure 30. From this graph, the F-numbers of the lens for which a light beam having an incidence angle corresponding to the 1 st order diffraction angle 0, begins to be kicked are shown in the table below.

Pitch of diffraction lattice F-number 11 ssm 10 10 9.1 9 8.2 8 7.3 7 6.4 6 5.5 Accordingly, as the pitch of the diffraction lattice becomes smaller, the F-number for which the incident light corresponding to the 1 st order diffraction angle is kicked becomes smaller and as a result, when the pitch of the diffraction lattice is very small, the focus indicating cannot take place even if a bright lens is used.

With the fact taken into account that the vicinity of f/5.6 is the available limit in the focusing screen using a conventional image splitting bi-prism, and in order that a performance superior to that of the conventional product may be provided with regard to the F-number of a lens with with the focusing screen of the present invention can be used, the pitch of the diffraction lattice provided on the focus indicating part should desirably be 7 ym or greater. However, the desirable lower limit value of the pitch of the diffraction lattice is restricted also by still another factor. That is, where the pitch of the diffraction lattice is made very small, the diffraction term has a more widened characteristic and therefore, the 0order diffracted light which does not contribute to the focus indicating becomes very great in quantity.

Particularly, where the diffraction lattice used is a phase diffraction lattice of sawtooth-like crosssection, the width P2 of the minute prism portion having an inclined surface having a small angle of inclination 02, such as 21, 27 in Figure 16 or 54 in Figure 22B, also becomes very small and the contribution 61 (Figure 24) to the diffraction term by this minute prism portion has a widened characteristic and therefore, if the configuration of the lattice is designed so as to secure some extent of quantity of diffracted light for the 1 st order diffracted light, an innegligible quantity of O-order diffracted light will be produced.

Design Example 5: P = 10 m,P1 = P2 = 5 m,#1 = 12.7 ,02=6.4 The diffracted light distribution for white incident light in this design example is shown in Figure 31. If, as in this design example, the pitch of the prism-like unit structure, namely, the pitch of the diffraction lattice, is of the order of 10 ym, the O-order diffracted light becomes considerably intense and amounts to 1 5% of the total diffracted light.A value up to this value of the O-order diffracted light is the limit for maintaining the performance with which the focus indicating part can be effectively used and accordingly, when the aforementioned problem of the 1 st order diffracted light being kicked is taken into account, the pitch of the diffraction lattice should desirably be 10 4m or greater.

From what has been described above, it is seen that including the condition for which the light and-dark figure of the focus indicating part is not seen, the most preferable range of the pitch of the diffraction lattice provided o. the focus indicating part is 10 FLm ym < P < 50 um. Of course, this range is also preferable for the embodiments of Figures 2, 7, 8, 1 0A, 11, 16 and 18.

In the case of the embodiments shown in Figures 7 and 22C, the pitch of the diffraction lattice is coincident with the arrangement period of the prism-like unit structures and therefore, the arrangement period of the prism-like unit structures should also preferably satisfy said range.

Also, in the focus indicating part as shown in Figures 1 6, 1 7, 18 and 22 wherein the light deflecting portion is formad by repetition of two inclined surfaces having angles of inclination jr and 02t a good performance can be obtained by further satisfying the following condition.This is based on the fact that the diffracted light distribution can be controlled also by varying the angles 0, and 02. The diffraction term which brings about such diffracted light distribution is shown in Figure 24, and if the angles which should be the peaks of the contribution 60 from the inclined surface having the angle of inclination 0| to the diffraction term and the contribution 61 from the inclined surface having the angle of inclination 02 are OHi and 82 respectively, then tan 0 sin sin Oi (=1,2) n- 1 Because Xj and (e3j are small, there is approximately the following relation:: 1 n--l where n is the refractive index of the material forming the focus indicating part. Accordingly, by setting the angle of W (i=1, 2), the positions of the peaks of the contributions 60 and 61 of the diffraction term can be controlled.

In a focusing screen incorporated into a single lens reflex camera, in order that the focus indicating may be possible even for a lens having a small aperture, the position of the peak of the diffraction term 61 should desirably be 50 or less. This is because, if the peak position of the diffraction term 61 exceeds 50, the diffracted light having a small diffraction angle of 40 or less becomes small in quantity and the image on the focus indicating part dark-changes for an interchangeable lens having a small aperture.

Consequently, the condition that 5 #2 # n - 1 should desirably be satisfied. Where the material of the focusing screen is acryl having a refractive index n = 1.49, the upper limit value of 02 is about 100. A design example for 02 = 100 will be shown below.

Design Example 6: P = 24 ssm, Pa = P2 = 12 um, sb1 = 16.0 ,02 = 10.0 The diffracted light distribution for white incident light in this design example is shown in Figure 32.

As seen from Figure 32, the diffracted light having a diffraction angle of 40 or less is small in quantity and if 02 is a greater angle, the diffracted light having a smaller diffraction angle is decreased in quantity and the image on the focus indicating part becomes liable to dark-change for a lens having a small aperture.

Also, if the greater angle of inclination 0, is made too great, the peak position of the diffraction term 60 becomes deviated toward the greater diffraction angle. Therefore, the diffracted light having a great diffraction angle is increased to enhance the accuracy of the focus indicating, but the focus indicating part becomes darkened even when an interchangeable lens having a bright F-number is mounted. In the case of the ordinary single lens reflex camera, it is desirable that the focus indicating part be not darkened up to the order of F3.5. The diffraction ang;e O corresponding to this F-number is 8.20 and therefore, sX, should desirably satisfy the conditon that 8.20 #1 # - n- 1 Where n = 1.49, the upper limit value of sX, is about 160.

Design Example 6 is also a design example using this upper limit value of 0, and, in Figure 32 which shows the diffracted light distribution thereof, the diffracted light has a diffraction angle amounting to the vicinity of 11 0, but if s6, is further increased, the diffracted light having a greater diffraction angle is increased and the dark of the focus indicating part becomes more and more remarkable.

From the foregoing fact, it is desirable in order to obtain a good performance of the focus indicating part that the angles of inclination 01 and 02 be equal to or less than 8.20/n - 1 and 50/n - 1, respectively.

Next, the influence imparted to the diffracted light distribution by a great difference between the angles of inclination t and 02 will be shown in Design Example 7.

Design Example 7: P=4Om, ym, P, = P2 = 20,um, 15%=50 The diffracted light distribution for white incident light in this design example is shown in Figure 33. It is seen from Figure 33 that in this design example, the diffracted light having a diffraction angle 0 in the vicinity of 40--6 0 is small in quantity.In the light deflecting portion having such diffracted light distribution, even if the aperture diameter of the lens is varied from the condition in which the light beam having an incidence angle of 60 begins to be kicked until the light beam having an incidence angle 40 begins to be kicked, the light beam available for the focus indicating is only the light beam having an incidence angle of 40 or less and therefore, even if the aperture diameter is increased so that the maximum incidence angle of said incident light beam varies from 40 to 60, the accuracy of the focus indicating remains low.From such a point of view, it is desirable that the diffracted light be continuously distributed in a certain range of diffraction angle and accordingly, it is not preferable that the difference between the angles of inclination 0, and 02 is too great.

Figure 34 is a cross-sectional view of the light deflecting portion according to another embodiment of the present invention. In this embodiment, the light deflecting portion 70 comprises wedge type unit structures of two diffraction lattices provided on a Fresnel band of the image splitting prism, and if viewed from another standpoint, comprises piism-iike uriit structuresormed by three inclined surfaces having angles of inclination 0" j2 and 03 and arranged at a period P. Further, a light deflecting portion comprising prism-like unit structures formed by four or more inclined surfaces having different angles of inclination and periodically arranged is also applicable to the focusing screen of the present invention.Also, in the two light deflecting portions forming the focus indicating part according to the present invention, it is possible that the angles of inclination of the prisms and the diffraction lattices are different. Further, ip Figures 1 6 and 22B, it is not necessary that P, = P2, but if the width of the minute prism portion having a small angle of inclination is made greater, the diffracted light having a lower diffraction angle will be increased in quantity and, if the width of the minute prism portion having a great angle of inclination is made smaller, the diffracted light having a higher diffraction angle will be increased in quantity, whereby the characteristic of the focus indicating part may be varied.

To make the focus indicating part as described above, use may be made of a method which comprises applying photoresist onto a prism forming a conventional split image focus indicating part, making a concavo-convex structure by the photoetching technique, obtaining a metal mold from the concavo-convex structure by the electrocasting technique, and using this metal mold to transfer the concavo-convex structure onto plastics.

The focus indicating part of the form shown in Figures 22A, 22B and 22C may be made by rendering the edge shape of a diamond tool 80 used during the making of a metal mold into a shape as shown in Figure 35, core-lining a metal mold 81 by the diffraction lattice core-lining technique, and making a number of plastic copies from the metal mold 81. Particularly, the focus indicating part of this type is also suited for the injection molding because it has a small degree of surface roughness as compared with the conventional image splitting bi-prism.

The above embddiments have been shown with respect to an example in which there are two light deflecting portions in the focus indicating part, and various examples of application thereof will be shown hereinafter.

Figure 36A is a plan view of a focusing screen 94. Designated by 92 and 93 are focus indicating parts of the hitherto described cross-section which will cause lateral deviation of the image. Portions 90 and 91 have a construction as obtained by rotating the focus indicating parts 92 and 93 by 900 about the center axis.

Accordingly, when the focus indicating is effected with respect to the reticule 95 on an object by the use of these focus indicating parts, a blurred image as shown in Figure 36B will be viewed in defocus condition.

Description will now be made of an embodiment in which the direction of the prism forming the light deflecting portion and of the diffraction lattice provided thereon is made to differ from that in the hitherto described embodiments by 900. For example, in the light deflecting portion of Figure 2, only the diffraction lattice 5 of Figure 3 is rotated by 900 with respect to the optical axis, whereafter it is made integral with the prism 4. The split image by the prism portion can be utilized with respect to the longitudinal line, and the double image by the diffraction lattice can be utilized for the focusing with respect to the lateral line, thereby overcoming the disadvantage that with the conventional split image focus indicating part, the focusing is difficult with respect to a line parallel to the prism borderline.

As will be seen in the present embodiment, the minute structure on the split prism is not limited in direction even if it is one-dimensional, and accordingly, it may also be made into two-dimensiona! construction.

Other examples of application are shown in Figures 37A, 38A and 39. Figure 37A shows an example in which the light deflecting portions shown in Figure 1 7 are provided in the four areas 100, 101, 102 and 103 of the focus indicating part in different directions. Cross-sectional views along A-A' and B-B' are shown in Figure 37C. Figure 37B shows the~image in defocus condition when the reticule has been observed by the use of this focusing screen 104. Figure 37D shows the reticule image when the cross-section along B-B' is made into the same shape as the cross-section along A-A'.

In Figure 38A, the directions of the lattice lines are orthogonal to each other in inner areas 112 and 113 and outer areas 110 and 111. Also, in the areas 110 and 111, the directions of the prisms are opposite to each other. This also holds true with the areas 112 and 113. The reticule image in defocus condition in this focusing screen is shown in Figure 38B.

In the focusing screen shown in Figure 39, inner areas 120 and 121 are light deflecting portions having a diffraction lattice, and an outer area 122 is a conventional microprism range finder portion.

In view of the fact that the split image type focus indicating system generally has a disadvantage that it encounters difficulties in focus indicating for an object of a lateral line pattern, the embodiments shown in Figures 36A-39 provide a focusing screen which overcomes such disadvantage. Accordingly, the focusing screen of these embodiments can accomplish accurate focus indicating for a linear image in any direction.

The focusing screen of the present invention is useful not only in cameras, but also in various optical instruments.

Claims

1. In or for a coincidence type range finder system having an objective lens for forming an image plane, a focusing screen comprising: a first light deflecting portion having a first prism on which a diffraction lattice is formed, said first light deflecting portion deflecting an incident light mainly in a first direction; and a second light deflecting portion having a second prism on which another diffraction lattice is formed, said second light deflecting portion deflecting another incident mainly in a second direction; said first and second prisms being oriented oppositely to each other, said first and second directions being different from each other.

2. A focusing screen according to claim 1, wherein said first and second diffraction lattices are of the relief and phase type.

3. A focusing screen according to claim 2, wherein said diffraction lattices have pitches which are smaller than and equal to 50 um and greater than and equal to 10 ym.

4. A focusing screen according to Claim 2, wherein said first and second prisms are Fresnel prisms.

5. A focusing screen according to Claim 4, wherein a unit structure of said diffraction lattice is mounted on a strip portion of said Fresnel prism.

6. A focusing screen according to Claim 2, wherein said diffraction lattices are of the rectangular type.

7. A focusing screen according to Claim 2, wherein said diffraction lattices are of the sawtooth type.

8. In or for a coincidence type range finder system having an objective lens for forming an image plane, a focusing screen comprising: a plurality of light deflecting portions in each of which members of prism-like unit structure are arranged with a period "P" in the direction different from the other, said prism-like unit structure, in the cross-section thereof, having the surface thereof formed by a plurality of inclined surfaces having different angles of inclination with respect to the image plane, said period "P" being greater than 10 ym and less than 50,um.

9. A focusing screen according to Claim 8, wherein the smallest of the angles of inclination which the plurality of inclined surfaces forming the surface of said prism-like unit structure forms with the image plane is less than about 50 and the greatest of said angles of inclination is less than about 160.

10. A focusing screen according to Claim 8, wherein a pair of said light deflecting portions are semicircular and together form a focus indicating part of circular area, and said members of prism-like unit structure of said pair of light deflecting portions are oriented oppositely to each other.

11. A focusing screen according to Claim 8, wherein four light deflecting portions each are a quarter of a circle and together form a focus indicating part of circular area, and two adjacent light deflecting portions have their respective members of prism-like unit structure oriented in directions different from each other by 900.

12. A focusing screen according to Claim 10 or 11, wherein a number of microprisms are arranged on the outer peripheral portion of said focus indicating part.

13. In a single lens reflex camera having means for selectively mounting interchangeable objective lenses having different relative aperture ratios, a focus indicating system comprising: a focusing screen having a focus indicating part which has a pair of light deflecting prisms oriented oppositely; and means for supporting said focusing screen in a plane in coincidence with a plane in the view finder of said single lens reflex camera; a diffraction lattice being formed on each of said pnsm's, the image on the focus indicating part being split and blurred in the out of focus condition.

14. A focusing screen substantially as hereinbefore described with reference to the accompanying drawings.

1 5. A coincidence type range finder system having a focusing screen substantially as hereinbefore described with reference to the accompanying drawings.

1 6. A single lens reflex camera having a focus indicating system including a focusing screen substantially as hereinbefore described with reference to the accompanying drawings.