JP2001215608A - Branching optical element for digital single-lens reflex camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a branching optical element for a digital single-lens reflex camera(SLR) which allows the reflection and transmission at a specified ratio over the entire visible region relating to incident light having an incident angle of a prescribed width. SOLUTION: This branching optical element 2 for a digital SLR having a solid-state image pickup element for forming the electric signal corresponding to the light quantity of a luminous flux S3 made incident via a photographic lens group 1 branches the exit light S1 emitted from the photographic lens group 1 into at least reflected light S2 to be introduced toward a finder optical system direction and transmitted light S3 to be introduced toward a solid-state image pickup element direction. The branching optical element 2 described above has two rectangular prisms 21 and 22 and a joint part formed by grasping and joining multilayered dielectric films at the each other's slants of the two rectangular prisms 21 and 22 and is so constituted that the reflectivity for a visible region and the prescribed wavelength region expanded to a long wavelength side and short wavelength side from the visible region in the luminous fluxes advancing rectilinearly in parallel to the optical axis and entering the joint part at a prescribed angle among beams of the exit light from the photographic lens group is approximately constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a branch optical element used for a single-lens reflex digital camera.

[0002]

2. Description of the Related Art In recent years, a digital camera which converts an image photographed by a solid-state image pickup device such as a CCD into an electric signal and records it on a memory, a floppy disk or the like instead of a silver halide film has become widespread. Among digital cameras, single-lens reflex cameras, like cameras using silver halide film, can directly observe the image of a subject formed by a photographic lens with a viewfinder, so there is no spatial parallax. Have advantages.

[0003] As an example of a conventionally known single-lens reflex digital camera, a branch optical element having a half mirror surface on which a large number of dielectric thin films (hereinafter, referred to as a multilayer film) is laminated is provided. There is a type in which a light beam transmitted through a photographing lens is branched, and one light beam is guided to a solid-state imaging device serving as an imaging surface, and the other light beam is guided to a finder optical system. The multilayer film is designed so as to obtain an optimal reflection (or transmission) characteristic with respect to a component in a visible region of a light beam incident on the half mirror surface at a predetermined angle.
In this specification, the visible region is defined and described as a region from about 400 nm to about 700 nm.

FIG. 1 is a diagram showing a schematic configuration of a photographing optical system 10 'conventionally used in a single-lens reflex digital camera. The photographing optical system 10 'includes a photographing lens group 1, a beam splitter 2', and a CCD 3. Beam splitter 2 '
Is provided with a half mirror surface 2α ′. The half mirror surface 2α ′ is a surface provided with a multilayer film designed so that the reflectance of a light beam incident at a predetermined angle (here, 45 degrees) in the visible region is optimal (30%). It is. FIG. 2 is a graph showing the relationship between the wavelength (horizontal axis, unit: nm) of the light beam incident on the beam splitter 2 ′ and the reflectance (vertical axis, unit:%).

As shown by the solid line in FIG. 2, when the light beam enters the half mirror surface 2α 'at 45 degrees, the light beam in the visible region is reflected at a reflectance of 30% due to the characteristics of the multilayer film. However, the light from the subject not only travels in a straight line parallel to the optical axis of the photographing lens group 1 but also travels straight with an inclination of about 10 degrees with respect to the optical axis and enters the half mirror surface 2α ′. Luminous flux is included. Therefore, the incident angle of the light actually transmitted through the photographing lens group 1 on the half mirror surface 2α ′ has a width of about 45 ± 10 degrees. In FIG. 2, the dotted line represents the wavelength-reflectance characteristic of the light beam incident on the half mirror surface 2α ′ at 35 °, and the dash-dot line represents the wavelength-reflectance characteristic of the light beam incident on the half mirror surface 2α ′ at 55 °. I have.

When the angle of incidence of the light beam on the half mirror surface 2α ′ changes from a predetermined angle, the wavelength-reflectance characteristic of the multilayer film also changes. Specifically, when the angle of incidence on the half mirror surface 2α ′ is smaller than a predetermined angle, the region where the reflectance is constant (also 30%) shifts to the longer wavelength side. For example, in FIG. 2, it can be seen that the dotted line is shifted to a longer wavelength side (the right side in FIG. 2) than the solid line. When the angle of incidence on the half mirror surface 2α ′ is larger than a predetermined angle, the constant reflectance region shifts to the shorter wavelength side. For example, in FIG. 2, it can be seen that the dashed line is shifted to a shorter wavelength side (left side in FIG. 2) than the solid line. The above-described characteristics become more conspicuous as the difference between the incident angle of the light beam and the predetermined angle at the time of designing the multilayer film increases.

Here, when the region where the reflectance is constant shifts to the long wavelength side or the short wavelength side, the reflectance in the visible region of the light beam does not become constant, and the visible region of the light beam reflected by the beam splitter 2 'is not reflected. A phenomenon occurs in which the amount of light is also different. This phenomenon causes a part of an image photographed by the CCD 3 and a part of an image observed by a finder (not shown) to be reddish or bluish. For example, for a light beam incident on the half mirror surface 2α ′ at 55 degrees, the reflectance near 700 nm decreases as shown by the dashed line in FIG. Therefore, a phenomenon occurs in which an image formed by the light flux reflected by the beam splitter 2 ′ has a color different from the color of the subject that can be actually observed with the naked eye.

Conventionally, with respect to the above-mentioned phenomenon of an image photographed by the CCD 3, various correction processes are performed by improving the performance of an image processing unit provided in a camera, and an image displayed on an LCD monitor or the like is visually observed. At the stage of doing it, it was corrected to the extent that he was not bothered. However, not only does the cost increase due to the high performance of the image processing unit, but also the image processing takes time, so when checking the subject using the LCD monitor, the movement of the subject can be followed in real time. It may disappear.

Moreover, the image observed by the finder is an image obtained by forming the light beam reflected by the beam splitter 2 'as it is, and therefore cannot be corrected.
Therefore, the user has to determine the composition while viewing the image in which the above phenomenon occurs. This can be uncomfortable for the user.

[0010]

In view of the above circumstances, the present invention can reflect and transmit incident light having an incident angle having a predetermined width at a constant rate over the entire visible range. It is an object of the present invention to provide a branch optical element in a single-lens reflex digital camera.

[0011]

According to the present invention, there is provided a single-lens reflex camera according to claim 1, wherein the branching optical element is a solid-state imaging device for generating an electric signal corresponding to a light amount of a light beam incident through a photographing lens group. In the single-lens reflex digital camera provided, the light emitted from the photographing lens group is at least a reflected light guided to the finder optical system and a branched optical element that is branched to transmitted light guided to the solid-state imaging device, A right-angle prism, having a junction where the dielectric multilayer film is sandwiched and joined on the mutually inclined surfaces of the two right-angle prisms, of the light emitted from the photographing lens group, goes straight in parallel with the optical axis; In the light beam incident on the joint at a predetermined angle,
It is characterized in that the reflectance in a visible wavelength range and a predetermined wavelength range expanded from the visible wavelength range to a longer wavelength side and a shorter wavelength side is a substantially constant value.

According to the present invention, due to the structure of the single-lens reflex digital camera, even if the incident angle of the light beam incident on the branching optical element has a predetermined width, the reflectance over the entire visible region can always be made uniform. With the finder optical system and the solid-state imaging device, an image can be obtained which has high image quality and does not cause a phenomenon in which a color different from the color of a subject which can be actually observed with the naked eye is applied.

[0013] Specifically, the above-mentioned predetermined wavelength range is set so that even if the luminous flux included in the light emitted from the photographing lens group enters the joint at an incident angle different from the above-mentioned predetermined angle, The wavelength range is such that the reflectance of the luminous flux over the entire visible range is always substantially constant (claim 2).

It is desirable that the dielectric multilayer film is formed by alternately stacking at least two types of films of a low refractive dielectric and a high refractive dielectric (claim 3). It is preferable that the low refractive dielectric has a refractive index n _L of 1.30 <n _L <1.66, and the high refractive dielectric has a refractive index n _H of 1.90 <n _H <2.50 (claim 4).

According to the branch optical element in the single-lens reflex camera of the present invention, when the dielectric multilayer film has low refractive dielectrics L1 to L6 and high refractive dielectrics H1 to H7, the photographic lens H1 / L1 / H2 / L2 / H3 / L3 / H4 / L4 / H in the order in which the light beams emitted from the group are incident
5 / L5 / H6 / L6 / H7, and the film thickness (QWOT) of each dielectric is 143 nm <H1 film thickness <175 nm 172 nm <L1 film thickness <211 nm 1304 nm <H2 film thickness <1594 nm 182 nm < L2 film thickness <223 nm 204 nm <H3 film thickness <249 nm 463 nm <L3 film thickness <565 nm 928 nm <H4 film thickness <1134 nm 450 nm <L4 film thickness <550 nm 835 nm <H5 film thickness <1021 nm 464 nm <L5 Film thickness <567 nm 498 nm <H6 film thickness <609 nm 179 nm <L6 film thickness <218 nm 480 nm <H7 film thickness <586 nm.

[0016]

FIG. 3 is a diagram showing a schematic configuration of a photographing optical system 10 in a single-lens reflex digital camera having a beam splitter 2 according to the present invention. 3, the same members as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. The photographing optical system 10 includes a photographing lens group 1, a beam splitter 2, and a CCD 3 in the order in which light from a subject is incident.

The photographing lens group 1 converts light from a subject into CC light.
An image is formed on D3. Light S emitted from the photographing lens group 1
1 enters a beam splitter 2, which splits the light S1 into a reflected light S2 guided to a finder optical system (not shown) and a transmitted light S3 guided to a CCD3. The CCD 3 captures an image by receiving the transmitted light S3 and transmitting an electric signal to an image processing unit (not shown).

FIG. 4 is an enlarged view of the beam splitter 2. As shown in FIG. 4, the beam splitter 2 has two right-angle prisms 21 and 22 and a joint 2α.

The joint 2α is formed on the inclined surface 2 of the right-angle prism 21.
1α, the slope 22α of the right-angle prism 22 and the multilayer film 23. The bonding portion 2α is bonded to the opposing surfaces of the multilayer film 23 with a predetermined adhesive (for example, a refractive index n = 1.49) in a state where the slopes 21α and 22α are parallel to each other. Accordingly, the beam splitter 2 has a substantially rectangular parallelepiped shape as a whole, and is joined in the order of the right-angle prism 21, the multilayer film 23, and the right-angle prism 22 when viewed from the photographing lens group 1.

The multilayer film 23 is formed by laminating a plurality of at least two types of dielectrics. In FIG. 4 which is an enlarged view, for convenience of explanation, the multilayer film 23 is drawn so as to have a thickness. The beam splitter 2 is arranged so that the angle formed by the optical axis of the photographing lens group 1 and a normal established at a point where the optical axis intersects the slope 21α is 45 degrees. At the joint 2α, the beam splitter 2 reflects the incident light S1 at an optimum ratio to the reflected light S2 guided to the finder optical system and the reflected light S2 to the finder optical system.
The light is branched into transmitted light S3 guided to D3.

As described above, the photographing lens group 1
The light S1 emitted from the camera is not limited to a light beam that travels straight in parallel with the optical axis of the photographing lens group 1. In the following text, for convenience, three light beams included in the light S1, the light S2, and the light S3 will be described in detail. First, a light beam that travels straight parallel to the optical axis of the photographing lens group 1 is assumed, and the light beam is referred to as a light beam A. next,
Assuming a light beam having the minimum angle of incidence (35 degrees) on the slope 21α and a light beam having the maximum angle of incidence (55 degrees) on the slope 21α, the former is the light beam B and the latter is the light beam C. (FIG. 3). That is, the image formed by the light beam A is the central portion of the image, and the image formed by the light beam B and the light beam C is the peripheral portion of the image. The angle of incidence refers to an angle formed by the center line of the light beam and a normal line set at a point where the center line intersects the slope 21α.

The multilayer film 23 is such that the light flux A, the light flux B, and the light flux C having different angles of incidence on the inclined surface 21α have a constant optimum reflectance and transmittance throughout the visible region. In the present text, the above-mentioned optimum values are a reflectance of 30% and a transmittance of 70%. Specifically, the multilayer film 23 has characteristics as shown in FIG.

FIG. 5 is a table showing the relationship between the wavelength (horizontal axis, unit nm) and the reflectance (vertical axis, unit%) of the light beam A incident on the inclined surface 21α of the beam splitter 2 according to the present invention at an incident angle of 45 degrees. It is the graph which did. As shown in FIG. 5, the reflectance of the light flux A is substantially constant at 30% in a region where the visible region is expanded to the long wavelength side and the short wavelength side. Accordingly, the wavelength-reflectance characteristic of the light flux B or the light flux C shifts to the long wavelength side or the short wavelength side, but is always 30% in the visible region.
Can be maintained.

Hereinafter, embodiments of the present invention will be described.

One example of the configuration of the multilayer film 23 in the present embodiment is as follows.
An example is shown in Table 1. However, in the configuration of Table 1, G₂₁Is straight
The glass that is the material of the square prism 21 is represented by G₂₂Is a right angle
Represents the glass that is the material of the rhythm 22. H1 to H7
Represents a dielectric material having a high refractive index, and L1 to L6 represent low refractive indices.
Represents a dielectric material having a ratio. The film number is that light S1
They are given in the order of incidence. Each of the dielectrics H1 to H7,
And the dielectrics L1 to L6 are the same material, respectively.
No need. The refractive index of each of the dielectrics H1 to H7
Each n_H1~ N_H7Then, the refractive index n_HX(However, X
Is any natural number from 1 to 7) is 1.90 <n_HX
<2.50 must be satisfied, and each of the dielectrics L1 to L6 has
Is n._L1~ N _L6Then, the refractive index n
_LY(However, Y is any natural number from 1 to 6)
Is 1.30 <n_LY<1.66 must be satisfied.

[0026]

[Table 1]

In Table 1, the thickness of each dielectric (QWOT)
Must satisfy the following conditional expressions. 143 nm <H1 film thickness <175 nm 172 nm <L1 film thickness <211 nm 1304 nm <H2 film thickness <1594 nm 182 nm <L2 film thickness <223 nm 204 nm <H3 film thickness <249 nm 463 nm <L3 film thickness <565 nm 928 nm < H4 film thickness <1134 nm 450 nm <L4 film thickness <550 nm 835 nm <H5 film thickness <1021 nm 464 nm <L5 film thickness <567 nm 498 nm <H6 film thickness <609 nm 179 nm <L6 film thickness <218 nm 480 nm <H7 Film thickness <586 nm

As a specific example, a multilayer film 23 having a structure as shown in Table 2 below can be considered.

[0029]

[Table 2]

In the configuration of the multilayer film 23 shown in Table 2, as a dielectric having a high refractive index (H1 to H6 in Table 1),
Tantalum pentoxide (Ta ₂ O ₅ ) having a refractive index of 2.055 is used. Also, a dielectric material having a low refractive index (L1 to L in Table 1)
5) As magnesium fluoride with a refractive index of 1.388 (Mg
F ₂ ) and silicon dioxide (SiO ₂ ) with a refractive index of 1.4693 are used. In addition, it is assumed that the absorptance k of all the dielectrics constituting the multilayer film 23 shown in Table 2 is 0, and the attenuation of the light amount by the multilayer film 23 is not taken into consideration.

FIG. 6 shows the results of the multi-layer film 23 shown in Table 2.
It is a graph showing the characteristic with respect to the light beam A incident at an incident angle of 45 degrees. The vertical axis represents the transmittance and the reflectance of the light beam A in the multilayer film 23, and the horizontal axis represents the wavelength range of the light beam A in the multilayer film 23. In FIG. 6, a line T ₄₅ indicates a change in transmittance according to the wavelength of the light beam A, and a line R ₄₅ indicates a change in reflectance according to the wavelength of the light beam A.

As shown in FIG. 6, the multilayer film 23 has a light flux A
Has a reflectance of about 30% (line R ₄₅ ) over a wide wavelength range from about 360 nm to about 750 nm in which the visible range is expanded to both the long wavelength side and the short wavelength side (line R ₄₅ ), and about 70% (Line T ₄₅ ). As described above, the single-lens reflex digital camera according to the present embodiment employs a configuration in which the light S2 reflected by the beam splitter 2 is guided to a finder optical system (not shown). Therefore, in the visible region of the light beam A, about 30% is reflected and guided to the finder optical system, and about 70% is transmitted and guided to the CCD 3 by the action of the multilayer film 23 shown in Table 2.

The reason why the amount of light applied to the finder optical system is smaller than the amount of light applied to the CCD 3 is that the amount of light guided to the finder optical system is an amount necessary for determining the composition of the subject by the eyepiece of the finder optical system. It is enough. Conversely, since the CCD 3 transmits a signal corresponding to the amount of light received on the light receiving surface to an image processing unit (not shown), the CCD 3 gives as much light as possible.

The multilayer film 23 shown in Table 2 has a substantially optimum ratio (reflectivity of about 30% and transmittance of about 70%) in the wide wavelength range from about 360 nm to about 750 nm when the light beam A is incident.
%) (FIG. 6), the light flux B and the light flux C
, The characteristics shown in FIG.

FIG. 7 is a graph showing characteristics of the multilayer film 23 shown in Table 2 with respect to the light flux B and the light flux C. The vertical axis represents the transmittance and the reflectance of the light beam B and the light beam C incident on the multilayer film 23, and the horizontal axis represents the wavelength range of the light beam B and the light beam C incident on the multilayer film 23. A line T ₃₅ represents a change in transmittance according to the wavelength of the light beam B, and a line R ₃₅ represents a change in reflectance according to the wavelength of the light beam B. Also, the line T
Numeral ₅₅ represents a change in transmittance with respect to the wavelength of the light flux C.
Reference numeral ₅₅ denotes a change in the reflectance depending on the wavelength of the light flux C.

As shown in FIG. 7, the multilayer film 23 has an incident angle
Even for the light flux C of 35 degrees, in the wavelength range from about 380 nm to about 750 nm including the visible range, about 30% is reflected almost constantly (line R ₃₅ ), and about 70% is transmitted (line R ₃₅ ). T
₃₅ ). Similarly, the multilayer film 23 reflects the light beam C at an incident angle of 55 degrees in a wavelength range from about 350 nm to about 720 nm including the visible light area, and reflects about 30% substantially uniformly (line R ₅₅ ). The remaining approximately 70% is transmitted (line T ₅₅ ).

Table 3 shows specific numerical values in FIGS. 6 and 7. Table 3 shows the reflectance and transmittance when the light flux A (incident angle 45 degrees), the light flux B (incident angle 35 degrees), and the light flux C (incident angle 55 degrees) are incident on the multilayer film 23 shown in Table 2. , The minimum value of 400 nm, the intermediate value of 550 nm, and the maximum value of 700 nm in the visible region. In Table 3, the reflection ratio and the transmission ratio represent the reflectance and the transmittance at other wavelengths when the reflectance and the transmittance at 550 nm, which are the wavelengths most sensitive to human eyes, are set to 1. It is.

[0038]

[Table 3]

First, as shown in Table 3, even if a light beam is incident at any incident angle, each wavelength (400 nm, 550 nm,
700 nm), the difference of the reflection ratio (transmission ratio) is 0.05
Within a few. In general, the human eye reacts to the phenomenon that if the reflection ratio (transmission ratio) at each wavelength of the incident light beam has a difference of about 0.1 or more, the image will be colored differently from the color of the subject that can be actually observed with the naked eye. Easier to do. Therefore, the difference between the wavelengths of the light flux A, the light flux B, and the light flux C can be said to be a difference within an allowable range where the above phenomenon of the image is not felt.

According to Table 3, the optimal value of the reflectance (transmittance) of the light flux A and the light flux B in the visible region is 30% (transmittance 70%) due to the characteristics of the multilayer film 23. It can be seen that the overall decrease is about 2% (that is, the transmittance increases about 2%). In addition, it can be seen that the reflectance of the light beam C in the visible region uniformly increases by about 5% as a whole with respect to the optimum value (that is, the transmittance decreases by about 5%).

As described above, the fact that the reflectance (transmittance) of each light beam for each wavelength is uniformly different from the optimum value means that the brightness of an image formed by each light beam is different, that is, the brightness of an image observed or the like. Is different depending on the location. However, in general, the human eye reacts sensitively to the phenomenon in which a color different from the color of an object that can be actually observed with the naked eye is used as described above. There is a property that no reaction occurs unless the light amount changes by about 10% or more. Therefore, there is no response to a change in lightness caused by a uniform change in the amount of light in the entire visible region by about 2 to 5%. That is, when the beam splitter 2 of the present invention is used, a change in brightness does not matter.

As described above, the multilayer film 2 shown in Table 2
The beam splitter 2 provided with the light source 3 has a slope 21 of the incident light beam.
Even if the incident angles to α are different from each other, the reflectance and transmittance of the incident luminous flux over the entire visible range can be maintained at almost optimum values, and the image photographed by the CCD 3 and the image observed by the finder In both cases, a high-quality image can be obtained without a phenomenon in which a color different from the color of the subject that can be actually observed with the naked eye is applied. That is, the configuration of the multilayer film 23 shown in Table 2 can be said to be one of the optimal configuration examples as the multilayer film having the characteristics shown in FIG.

The above is the embodiment of the present invention. The present invention is not limited to these embodiments, and various modifications can be made without departing from the gist.

The multilayer film 23 in the embodiment described above
Has a 13-layer film configuration as shown in Table 1,
The film configuration is merely an example. That is, even if a different dielectric material is used or the number of layers is changed, the above-described characteristics (see FIGS. 5 and 6) can be provided.

In this embodiment, the angle of incidence of the light flux A, which travels straight in parallel with the optical axis of the photographing lens group 1, on the slope 21 α is
This is a preferable value to make the camera body more compact.
The beam splitter 2 is arranged at 45 degrees.
However, the incident angle of the light beam A may be other than 45 degrees. For example, the beam splitter 2 can be arranged so that the incident angle of the light beam A on the multilayer film 23 becomes 35 degrees. In this case, the multilayer film 23 may have a reflection and transmission characteristic as shown in FIGS. 5 and 6 for light having an incident angle of about 35 degrees ± 10.

Further, in the present embodiment, the optimum values of the reflectance and the transmittance of the beam splitter 2 are set to 30% respectively.
And 70%, but these are all examples. Therefore, an optimum value can be determined according to the performance of the solid-state imaging device used in the single-lens reflex digital camera.

[0047]

As described above, the branching optical element in the single-lens reflex digital camera of the present invention can be applied to the entire visible range of the incident light beam even if the incident angle of the light beam incident on the junction has a certain width. By using a multilayer film that can be branched into light guided to the finder optical system and light guided to the solid-state imaging device at a fixed rate, the color of the object that is actually observable with the naked eye is It is possible to observe a high-quality image without the phenomenon of being obstructed by the finder optical system and to shoot by the solid-state imaging device.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of a photographing optical system of a conventional single-lens reflex digital camera.

FIG. 2 is a graph showing the relationship between the wavelength of an incident light beam and the reflectance of a conventional beam splitter.

FIG. 3 is a diagram illustrating a schematic configuration of a photographing optical system in a single-lens reflex digital camera having the beam splitter according to the embodiment.

FIG. 4 is an enlarged view of the beam splitter of the embodiment.

FIG. 5 is a graph showing the relationship between the wavelength of an incident light beam and the reflectance of the beam splitter of the present invention.

FIG. 6 is a graph showing characteristics of the multilayer film of the embodiment.

FIG. 7 is a graph showing characteristics of the multilayer film of the embodiment.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photographing lens group 2 beam splitter 21 right angle prism 22 right angle prism 23 multilayer film 2α joint 3 CCD 10 photographing optical system

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Isao Okuda 2-36-9 Maenocho, Itabashi-ku, Tokyo Asahi Gaku Kogyo Co., Ltd. F-term (reference) 2H042 CA10 CA14 CA18 2H054 AA01 CD00 5C022 AA13 AC02 AC09 AC78

Claims (7)

[Claims] 1. A single-lens reflex digital camera including a solid-state imaging device that generates an electric signal corresponding to the amount of light incident through a photographing lens group, wherein at least a finder optical system emits light emitted from the photographing lens group. A branched optical element that is branched into reflected light guided to the solid-state imaging device and transmitted light guided to the solid-state imaging device, wherein two right-angle prisms and the inclined surfaces of the two right-angle prisms narrow the dielectric multilayer film. And a joining portion joined and held, and of the emitted light, the reflectance for a light beam that goes straight in parallel with the optical axis and enters the joining portion at a predetermined angle has a visible wavelength range on the long wavelength side and A branch optical element in a single-lens reflex digital camera, wherein the optical element is substantially constant in a predetermined wavelength range expanded to a short wavelength side. 2. The method according to claim 1, wherein the predetermined wavelength range is such that a light beam included in the emitted light is incident on the junction at an incident angle different from the predetermined angle, and the wavelength-reflectance characteristic shifts. 2. The branch optical element in a single-lens reflex digital camera according to claim 1, wherein the reflectance of each light beam in the visible region is set to be substantially constant at all times. 3. The single-lens reflex camera according to claim 1, wherein the dielectric multilayer film is formed by alternately stacking at least two types of films of a low refractive dielectric and a high refractive dielectric. Branch optical element in digital camera. 4. The low refractive dielectric has a refractive index n _L of 1.30.
4. The branch optical element according to claim 3, wherein n _L <1.66, and the high refractive dielectric has a refractive index n _H of 1.90 <n _H <2.50. 5. 5. The dielectric multilayer film according to claim 1, wherein the low refractive dielectric is L
Assuming that the high refractive dielectrics are H1 to H7, the order of the incident light is H1 / L1 / H2 / L2 / H3 / L3 / H4 / L4 / H.
5 / L5 / H6 / L6 / H7, and the film thickness (QWOT) of each dielectric is 143 nm <H1 film thickness <175 nm 172 nm <L1 film thickness <211 nm 1304 nm <H2 film thickness <1594 nm 182 nm < L2 film thickness <223 nm 204 nm <H3 film thickness <249 nm 463 nm <L3 film thickness <565 nm 928 nm <H4 film thickness <1134 nm 450 nm <L4 film thickness <550 nm 835 nm <H5 film thickness <1021 nm 464 nm <L5 5. The single-lens reflex digital camera according to claim 3, wherein the thickness has a relationship of: 567 nm 498 nm <thickness of H6 <609 nm 179 nm <thickness of L6 <218 nm 480 nm <thickness of H7 <586 nm. Branch optical element in camera. 6. A single-lens reflex digital camera including a solid-state imaging device for generating an electric signal corresponding to the amount of light incident through a photographing lens group, the incident light being at least reflected light guided to a finder optical system. A branch optical element for branching into transmitted light guided to the solid-state imaging device, wherein two right-angle prisms are joined together by holding a dielectric multilayer film between inclined surfaces of the two right-angle prisms. Wherein the incident light has a predetermined width, and the incident light is reflected at a predetermined reflectance over the entire visible range of the incident light. 7. The branching optical element according to claim 6, wherein the incident angle is 45 degrees ± 10 degrees.