JPH0147771B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focusing plate for an optical machine such as a single-lens reflex camera.

As a conventional reticle of this type, one is known that is equipped with a split prism, in which a pair of wedge-shaped prisms in cross section with equal apex angles are combined in opposite directions; Since the lens aperture is constant, if the lens aperture is stopped down to F4 or higher, it becomes dark and becomes unusable.In addition, wide-angle lenses have the disadvantage that the apex angle of the prism is small, resulting in small image separation and the inability to obtain sufficient sensitivity. It was hot. For this reason, reticle lenses equipped with two types of split prisms with different apex angles are also known, but this makes it difficult to see the viewfinder field of view because the split prism with a large apex angle becomes dark when the lens aperture is closed down. However, since it has two split prisms, it cannot be placed in the center, so it has the disadvantage that it is not possible to focus at the center. In addition, some cameras have interchangeable focus plates that can be replaced depending on the intended use, but this requires the lens or pentaprism to be removed when replacing the focus plate, which is inconvenient in operation.

Therefore, the present invention is configured so that the apex angle of the split prism can be switched between two angles, large and small, so that it can be used without darkening or shadowing even when the aperture of the lens is stopped down. The purpose is to provide a focusing plate that can obtain sufficient sensitivity even in a wide-angle lens, and this will be explained using one embodiment shown in the drawings. In FIG. 1, 1 is the camera body, 2 is the lens, 3 is a pentaprism, 4 is a reflective mirror, and 5 is a focusing plate according to the present invention, which is arranged to be superimposed on a transparent upper stage 5a in the optical axis direction with respect to the upper stage 5a and to be movable relative to the optical axis in a direction perpendicular to the optical axis. It is composed of a transparent lower stage 5b. As shown in FIG. 2, in the center of the upper surface of the upper stage 5a, there are three prisms A, B, and C with large angles θ _1. Each prism surface forming an apex angle θ ₁ intersects the optical axis. The apex portions of the adjacent prisms having the apex angle θ ₁ are located on opposite sides of the line connecting the centers of the adjacent prisms, and the apex portions of the adjacent prisms are located on opposite sides of the line connecting the centers of the adjacent prisms. At the center of the lower surface, prism surfaces having a small apex angle θ ₂ and forming the apex angle θ ₂ are arranged in parallel so that each prism surface is included in a plane intersecting the optical axis, and the apex angle θ of the adjacent prism is Two prisms X and Y, whose apexes having a diameter of ₂ are located on opposite sides of each other with respect to a line connecting the centers of adjacent prisms, are arranged so as to face A and B or B and C by switching.

Since the focusing plate according to the present invention is constructed as described above, when narrowing down the aperture of the lens, prism X becomes prism A, prism Y becomes prism B, and so on.
If the prisms are made to face each other, a split prism having a small apex angle as a whole, where the apex angle of the prism is (θ ₁ −θ ₂ ), is constructed (see FIG. 3).
In this case, when the focus position is at point P, the image is observed at point Q on the optical axis. In this way, this focusing plate does not become dark or shaded even when the lens aperture is stopped down. In addition, when using _a wide _- angle lens, if prism A split prism is constructed (see Figure 4). In this case, when the focus position is at point R, the image is observed at point S on the optical axis. On the other hand, when the focus position is at point T, which is in front of point R (see FIG. 5), the image is observed at point U, which is offset from the optical axis, and the image of the split prism is shifted. In this way, even when a wide-angle lens is used, the present focusing plate has a large image separation and can provide sufficient sensitivity.

In the above embodiment, the focus positions P and R in FIGS. 3 and 4 are different from each other, but if this difference is large compared to the depth of focus, it will cause a problem in practice.
In order to avoid this problem, each prism A, B, C, It is also possible to reduce the thickness in the optical axis direction, thereby reducing the amount of movement of the focus position due to switching, and keeping it within the depth of focus.

As described above, the present invention has the effect that it can be used without darkening or shadowing even when the aperture of the lens is narrowed down, and that sufficient sensitivity can be obtained even with a wide-angle lens. Even when switched, the split prisms are configured in a symmetrical shape, making it very easy to see.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a single-lens reflex camera incorporating a focus plate according to the present invention, and FIG. 2 is a perspective view showing the configuration of a split prism portion of the focus plate according to the present invention.
FIG. 3 is a sectional view when the reticle according to the present invention is replaced with a split prism with a small apex angle, and FIG. 4 is a sectional view when the reticle according to the invention is replaced with a split prism with a large apex angle. FIG. 5 is a sectional view showing the same state as FIG. 4 when the focus is out of focus, and FIG. 6 is a perspective view similar to FIG. 2 showing another embodiment of the present invention. 1... Camera body, 2... Lens, 3... Pentaprism, 4... Reflection mirror, 5... Focus plate,
A, B, C, X, Y...prism.

Claims (1)

[Scope of Claims] 1. At least three first prisms having a first apex angle are arranged so that each prism surface forming the first apex angle is included in a plane intersecting the optical axis. the first prism of the adjacent first prism
a first prism group in which the apexes having an apex angle of are located on opposite sides of each other with respect to a line connecting the centers of the adjacent first prisms, and having a second apex angle different from the first apex angle; At least two second prisms are arranged in parallel such that each prism surface forming the second apex angle is included in a plane intersecting the optical axis, and
and a second prism group in which the apexes of the prisms having the second apex angle are located on opposite sides of each other with respect to a line connecting the centers of the adjacent second prisms, which are arranged in an overlapping manner in the optical axis direction. The apex angle of the split prism can be adjusted by selectively causing one prism of the second prism group to face one or the other of two adjacent prisms of the first prism group. Focus plate that can be switched to two angles: large and small. 2. The focusing plate according to claim 1, wherein the first prism and the second prism each include a plurality of small prisms.