GB2128050A - Arrangement for automatic focusing of photographic cameras - Google Patents

Abstract

A focusing arrangement for photographic cameras is provided wherein the brightness distribution of the exit pupil 3 of the entire system, the distribution being produced by the focus-governed shading effect, is evaluated in a measuring plane 9, the measured values being utilized for fully automatic or semi-automatic adjustment of the taking objective 2. The measuring plane lies in a second image plane 9, while in that plane of the pupil 8, which is situated between the first (4) and second (9) image planes, optical means 7 are provided for the production of at least two identical images 10a, 10b of a limited section of a first image plane 4 in the second image plane 9. The identical images 10a, 10b of the second image plane 9 are decomposable by means of a multiple array 10 into at least two partial images each, while from the brightness distribution of diametric pupil zones 7a, 7b the magnitude signal is obtained by comparing the receivers associated with the same parts of an image but corresponding to different partial images. The directional signal for the deviation in focus is obtained by taking into account the comparison of the receivers within a partial image. <IMAGE>

Description

SPECIFICATION Arrangement for automatic focusing of photographic cameras The invention relates to an arrangement for automatic focusing of photographic cameras, wherein the brightness distribution of the exit pupil of the entire system, the distribution being produced by the focus-governed shading effect, is evaluated in a measuring plane.

Methods are known in which the shading effect of an image field diaphragm in the exit pupil of a system is picked up photoelectrically by measuring the brightness distribution.

Methods are also known in which the image of the object to be focused is subdivided into partial images, the pupils thus being multiplied, the photoelectrical detection of the measured values being performed by multiple receiver arrays in the plane of the separate pupils. The decisive advantages of the image subdivision reside in the possibility of obtaining a signal of the correct sign, at the same time increasing the sensitivity.

There are nevertheless also serious drawbacks.

The resolving power of the image subdivision and thus the sensitivity of the system are restricted by the limitations affecting the production of the necessary optical means for the said image subdivision. This results in unsatisfactory adaptation of the multiple receivers, which owing to the greater dimensions required for the said receivers are unable to operate at the optimum ratio of useful operation to interference, while on the other hand they are technologically producible to considerably smaller dimensions, thus allowing more satisfactory ratios of effective to disturbed operation.

By way of explanation it should be noted that the focus-governed measuring effect (relative alteration of focus signal in the event of defocusing by a fixed amount) increases in direct proportion to the reduction in the width of the partial image and that on the other hand the limit luminous density (minimum processable luminous density of object, according to contrast of object) decreases in inverse proportion to the root of the resolving power (partial images per mm).

The application of the invention is intended to eliminate the aforementioned disadvantages.

The purpose of the invention is to improve the focusing by increasing the sensitivity of the system and reducing the limit luminous density by means of an arrangement in which the resolving power of the image subdivision is mainly determined solely by the resolving power of the array of multiple receivers.

The invention solves this problem by an optical arrangement in which the measuring plane lies in a second image plane, while in that plane of the pupil which is situated between the first and second image plane optical means are provided for the production of at least two identical images of a limited section of a first image plane in the second image plane, the identical images of the second image plane being decomposable by means of a multiple array into at least two partial images each, while from the brightness distribution of diametric pupil zones the magnitude signal is obtained by comparing the receivers associated with the same parts of an image but corresponding to different partial images and the directional signal for the deviation in focus being obtained by taking into account the comparison of the receivers within a partial image.For the performance of the automatic focusing operation it is also necessary in the plane of the pupil, in diametric peripheral parts of the pupil, for at least two pupil lenses in a first direction and/or in a second direction perpendicular thereto. If the pupil lenses are positioned in one direction only, i.e. if measured values are only processed in one direction, then this direction is preferably inclined at an angle a in relation to horizontal or vertical edges of the object. In an improved version of the arrangement according to the invention a central pupil lens (central image C), of which the optical axis coincides with the optical axis of the picture-taking lens, is provided in addition to the aforementioned diametric pupil lenses.The focal lengths of the pupil lenses are preferably dimensioned to ensure that in the second image plane a reduction will take place by comparison with the first image plane. As an example of an image limiting means an image field diaphragm is provided in the first image plane. It is also desirable for screening diaphragms to be provided between the images in the second image plane. For the practical application of the arrangement to which the invention relates it is of advantage for the optical components positioned in the first image plane and in the subsequent planes and also the multiple array to be combinable in the form of a compact 'building block'.For the processing of the measured values the focusing system is combined with a signal processing part in such a way that (m-1) first differences M1n of associated receivers of two images A and B from diametric pupil parts according to M1, = (An - Bn)n = 1.... (m-1 ) or M2n = {Bn + 1 - Bn)n = 1 ....(m-1 ) are formed and the signs M1n and M2n established, the sign of the focus deviation for each of the (m-i) focus signals being indicated by whether or not the signs of Mn1 and M2n agree.According to the invention it is of greater advantage for the signal M2n to be derived from a central image C which does not participate in the formation of the signal Min and which is multiplied by the signal M1n to obtain the signal. The signs of the signals M1n and M2n can also be tested by means of a logical circuit. A further characteristic of the arrangement according to the invention is that from m-1 focus signals at least one signal is further processed direct or at least two signals are further processed in summated form.

Example The invention will be explained below in greater detail by reference to an example. The drawings are as follows: Figure 1. A schematic diagram of the arrangement according to the invention.

Figure 2a, 2b. Alternative ways in which the pupil lenses can be arranged.

Figure 3. Conditions applicable to the images produced on the CCD array.

Figure 4. An example of the arrangement according to the invention in an SR camera.

In Figure 1 the taking lens 2 forms, in a first image plane 4 equivalent to the film plane, the image 6, limited by the image field diaphragm 4a, of the subject 1 to be focused. The measuring field lens 5 situated in the first image plane 2 projects into the pupil plane 8 the exit pupil 3 of the taking lens 2. In the pupil plane 8 the pupil lenses 7a, 7b and 7c are provided in the form of the variant illustrated in Figure 2a. They produce in the second image plane 9 three identical images 10a, 1 Ob and 1 Oc of the image 6 of the first image plane 4 on a multiple receiver array 10, e.g. a CCD array in accordance with Figure 3. Between the pupil lenses 7a and 7c and also 7c and 7b two screening diaphragms 1 Ia and 1 ib are provided.

The image sizes will be briefly illustrated by means of a numerical example: The image size 6 amounts, for example, to 4.8 x 4.8 mm2.

The aperture number kM of the measuring system follows from the distance dsA between the planes 8 and 4 and from the diameter d8a of the total pupil 8a. The measuring aperture number is kM = d8 4 ~ 3.4 8a if, for example, interchangeable objectives are to be used of which the aperture number for the open diaphragm is k0 = 2.8.

It follows that for ds.4 = 15 mm, for example, d8a = 4.4 mm, with a focal length of about 2mm for the pupil lenses 7a-7c. The image size on the CCD array will thus amount to about 0.74 x 0.84 mm2.

If, for example, a raster of 40 partial receivers per mm in the x andy directions for the CCD array 10 is taken as a basis, this results in about 30 x 30 partial images (30 receiver lines for each of 30 partial receivers). The orientation of the lines is then identical with that of the pupil lenses 7(y direction). The result is that the only edge positions in the subject which can be evaluated with maximum measuring sensitivity are those which are perpendicular to the orientation of the lines.

Optimum sensitivity in the two directions perpendicular to each other calls for the lens arrangement shown in Figure 2b and consisting, for example, of 5 pupil lenses, with an appropriate matrix structure for the CCD array (interrogation in x andy directions), for the processing of the measured values.

In contradistinction to Figure 3, the CCD array 10, in extreme cases, may also consist of one line, in which case, the partial receivers, where the distance covered in the x direction is concerned, may be given an appropriate width in the direction, or alternatively, in order to ensure an optimum energy balance, the said x direction may be more closely concentrated optically, in the known manner.

To obtain the total focus signal, independent focus signals corresponding to the partial images are first of all formed as regards their magnitude and their sign (direction of adjustment, As, required for the objective).

The total focus signal can then be the signal of a freely selectable individual partial image or made up of the focus signals of a number of partial images (maximum m-1) (summated). The criterion for the selection of certain partial images is the improvement of the yield of the system.

In order, for example, to eliminate a background causing interference (simultaneous presence of subjects at different distances) it may be desirable for only the nearest individual subject in each case to be selected by a computer program and further processed.

In this case, since the partial focus signals associated with the partial subjects situated at different distances pass through zero at different subjects positions, that partial focus signal is selected of which the passage through zero is nearest to the close-up adjustment limit.

The formation of the magnitude for each partial image is effected by comparing the diametrically opposite pupils 7a and 7b, i.e. by forming a partial image An and Bn belonging to the images 10a and 10b and associated with a first different M1n; M1n = (An - Bn} n = 1 (m-1) The partial signal of each partial receiver is thus determined by the integral image content of the relevant partial image (this being strictly valid in the case of focusing) and by the shading effect of its limit (diaphragm effect) transferred to the first image plane 4 in the case of defocusing.

As the difference Mien, as determined by the brightness sequence H/D or D/H in the image, causes no clear sign of the defocusing As, the direction has to be clearly determined. This is effected in Figure 10c by comparing the corresponding partial image Cn with the adjacent partial image, e.g. Cn + 1, in accordance with the formation of a second different.

M2n = (Cn + 1 - Cn} n = 1 (m-1) It is found that the change in the brightness sequence, e.g. from D/H to H/D, changes both the sign of the first difference M1n and the sign of the second difference M2n.

For this reason the sign of the focus signal is definitely positive if both differences are of the same sign and negative if they are of different sign or vice versa. This can be tested by a simple logical circuit. The same result is obtained if the two differences, M1n and M2n, are multiplied.

For the formation of the differences M2n Figure 1 0a or lOb can also be used, in which case the central pupil lens 7c and thus Figures 1 Oc are no longer required. It must be borne in mind, however, that with greater defocusings the sign may become incorrect as a result of shading and lateral image displacement.

This sign falsification is reduced sufficiently if the difference M2n is formed with the use of pupil parts as close as possible to the optical axis of the system.

For this reason the distance to which the pupil lens 7c extends in they direction should be as small as possible and the lens surfaces of the lenses 7a and 7b as far as possible equal.

In focusing the difference M1n passes through zero. The difference M2n, regardless of the defocusing, is always other than zero, provided there is sufficient contrast.

The measured-value processing described refers to a line in-they direction. If several lines are present they can be processed in succession. If an arrangement according to Figure 2b is used in conjunction with an x,y-matrix structure for the CCD array, then this processing method can also be adopted for the second direction.

For practical application processing in one direction is usually sufficient if the lens system 7a-7c and the associated array of receivers are orientated at an angle a = 45" in relation to horizontal and vertical edges of the subject.

The lens system should be arranged with a view to ensuring a compact construction and adequate measuring effects with the maximum possible reduction scale. When the focal length of the pupil lenses is made as small as possible attention must be paid to the lens diameter (f/d ratio), with a view to obtaining an image of the required quality. The f/d ratio appropriate to a short distance (d8.4 in Figure 1 ) is as high as possible.

Figure 4 shows an example for the optical arrangement in an SR camera.

The measuring device is accommodated in the floor of the camera in the known manner, the measuring rays taking their course through the partly translucent hinged mirror 4 with auxiliary mirror hinged thereto.

The optical components of the first image plane, of the pupil plane 8 and of the second image plane are combined to form a compact module 16. This is separately producible and balanced accordingly, the operation of building it into the camera being thereby greatly simplified. The individual designations are the same as those corresponding to Figure 1.

The solution provided by the invention allows of a smaller image than known arrangements, with 2 decisive advantages: The luminous intensity on the multiple array is thus the ratio of useful operation to interference are increased.

The system also enables a compact arrangement to be employed, which makes up for the need for a second image plane.

Furthermore, the partial receivers do not have to be adapted to the position determined by optical subdivision of the image. The photoelectrically defined partial image necessitates less adjusting and completely covers the measuring surface simply in accordance with the defects between the separate receivers.

Claims (13)

1. Arrangement for focusing of photographic cameras wherein the brightness distribution of the exit pupil of the entire system, the distribution being produced by the focus-governed shading effect, is evaluated in a measuring plane, the measured values being utilized for fully automatic or semi-automatic adjustment of the taking objective, characterized by the fact that the measuring plane lies in a second image plane (9), while in that plane of the pupil (8) which is situated between the first and second image plane optical means (7) are provided for the production of at least two identical images of a limited section of a first image plane in the second image plane (9), the identical images of the second image plane (9) being decomposable by means of a multiple array (10) into at least two partial images each, while from the brightness distribution of diametric pupil zones the magnitude signal is obtained by comparing the receivers associated with the same parts of an image but corresponding to different partial images and, the directional signal for the deviation in focus being obtained by taking into account the comparison of the receivers within a partial image.

2. Arrangement for focusing in accordance with Claim 1, characterized by the fact that in the plane (8) of the pupil, in diametric peripheral parts of the pupil, there are at least two pupil lenses (7a, 7b) in a first direction and/or in a second direction perpendicular thereto.

3. Arrangement for focusing in accordance with Claim 2, characterized by the fact that the pupil lenses (7a, 7b), are positioned in one direction only, this direction being preferably inclined at an angle in relation to horizontal or vertical edges of the subject.

4. Arrangement for focusing in accordance with Claim 3, characterized by the fact that a lens (7c) of which the optical axis coincides with the optical axis of the system is provided in the central part of the pupil plane (8).

5. Arrangement for focusing in accordance with Claim 4, characterized by the fact that the focal length of the pupil lenses (7) is such that in the second image plane a reduction takes place in relation to the first image plane.

6. Arrangement for focusing with a signal processing part in accordance with Claim 1, characterized by the fact that (m-1) first differences M1n of associated receivers of two images A and B from diametric pupil parts according to Min = {An - Bn} n = 1.... (m-1) or M2n = {Bn + 1 - Bn) n = 1....(m-1) are formed and the signs M1n and M2n established, the sign of the focus deviation for each of the (m-1) focus signals being indicated by whether or not the signs of Mn1 and M2n agree.

7. Arrangement for focusing in accordance with Claim 6, characterized by the fact that the signal M2n is derived from a central image C which does not participate in the formation of the signal M1n.

8. Arrangement for focusing in accordance with Claim 6, characterized by the fact that for the purpose of obtaining the correct sign the signals Min and M2n are multiplied or their signs are tested by means of a logical circuit.

9. Arrangementforfocusing in accordance with Claim 6, characterized bythefactthatfrom m-1 focus signals at least one signal is further processed direct or at least two signals are further processed in summated form.

10. Arrangement for focusing in accordance with Claim 1, characterized by the fact that image limiting means, such as an image field diaphragm (4a) are provided in the first image plane.

11. Arrangement for focusing in accordance with Claim 1,characterized by the fact that screening diaphragms (11 a, 11 b,) are provided between the images in the second image plane.

12. Arrangement for focusing in accordance with Claim 1, characterized by the fact that the optical components situated in the first image plane and in the subsequent planes and also the multiple array are combinable to form a compact building block (16).

13. An arrangementforfocusing of photographic cameras, substantially as herein described with reference to Figures 1 to 4 of the accompanying drawings.