JPH0625801B2 - Light splitting prism - Google Patents

Description

The present invention relates to a light splitting prism used in a single-lens reflex camera or the like.

In recent single-lens reflex cameras and the like, in order to split the focus detection light beam from the image formation light beam, for example, as shown in FIG. 1, the photographic objective lens 1 and the jumping mirror 2 (in FIG. (Shown) between the first prism 3 having a light splitting surface 3a and a second prism 4 having a surface 4a facing the light splitting surface 3a and joined to the light splitting surface 3a. With the splitting prism 5 arranged, most of the image-forming light flux emitted from the exit pupil 1a of the photographic objective lens 1 passes through the light-splitting surface 3a and is incident on the photosensitive surface 6, and a part of the image-forming light flux is As shown in FIG. 2, the light is reflected by the light splitting surface 3a and is guided to the focus detection element 7 as a focus detection light beam while repeatedly reflected in the first prism 3. However, as shown in FIG. 3, a part of the light beam (ghost light beam) that should be incident on the position P ′ outside the photosensitive surface 6 when it passes through the light splitting surface 3a is reflected by the light splitting surface 3a. Then, the light is totally reflected by the front surface 3b of the first prism 3, then transmitted through the light splitting surface 3a, then totally reflected by the rear surface 4b of the second prism 4, further reflected by the light splitting surface 3a, and then on the photosensitive surface 6. Since the light enters the position P ″, there is a problem that a ghost line is formed.

In view of the above problems, the present invention forms a thin air layer between a light splitting surface and a surface that faces the light splitting surface and is joined to the light splitting surface so that the ghost light flux is extracted out of the optical path by total reflection. A prism for use in the present invention will be described below. According to an embodiment shown in FIGS. 4 to 6, the light splitting surface 3a and the surface 4 facing and joined to the light splitting surface 3a will be described.
A sufficiently thin air layer 8 is formed between a and a. To form the air layer 8, for example, as shown in FIG. 5A, a spacer 9 made of foil or a thin vapor deposition film may be interposed between the light splitting surfaces 3a and 4a. in this case,
As shown in FIG. 5 (b), the spacer 9 is attached to the light splitting surface 3a.
By arranging it in the shaded portion B located outside the boundary line A of the effective light flux, the characteristics of the imaging optical system are not adversely affected.

The light splitting prism according to the present invention is constructed as described above, and since the air layer 8 is sufficiently thin, the behavior of the light beam transmitted through the light splitting surface 3a is the same as in the case of the conventional example (FIG. 1). Further, the behavior of the light flux which is reflected by the light splitting surface 3a and becomes the focus detection light flux is exactly the same as in the case of the conventional example (FIG. 2). However, when the ghost light flux reflected by the light splitting surface 3a is totally reflected by the front surface 3b of the first prism 3 and enters the light splitting surface 3a again, since the exit side of this surface 3a is the air layer 8, As shown in FIG. 4, the light is totally reflected and taken out of the optical path. The extracted ghost light flux does not enter the photosensitive surface 6 nor the focus detection element.
This will be described in detail. FIG. 6 is an enlarged cross-sectional view of an essential part of the first prism 3, in which a light ray 1 that should go to the outside of the photosensitive surface 6 when passing through the light splitting surface 3a.
Let i ′ be the refraction angle upon incidence on the optical axis, the reflection angle φ at the light splitting surface 3a is φ = α, where α is the apex angle of the first prism 3.
-i '. The reflection angle ω on the front surface 3b is ω =
2α-i ′, and the re-reflection angle δ at the light splitting surface 3a
Is δ = 3α-i ′. Therefore, at this time, in order that light does not pass through the light splitting surface 3a, it is sufficient to satisfy the condition of total reflection nsin δ ≧ n ₀ (a). Here, n is the refractive index of the first prism 3, and n ₀ is the refractive index of air. That is, the apex angle and the position of the first prism 3 may be determined so as to satisfy the expression (a).
Incidentally, in order to prevent the occurrence of aberrations, the incident surface of the prism is usually perpendicular to the optical axis. In this case ray l
The incident angle i of is the angle of view itself.

As described above, the light splitting prism according to the present invention has a very important advantage that the ghost light flux can be removed without changing its optical characteristics.

[Brief description of drawings]

FIG. 1 is a sectional view showing the behavior of a transmitted light beam in a conventional light splitting prism arranged in a single-lens reflex camera, and FIGS. 2 and 3 are sectional views showing the behaviors of a reflected light beam and a ghost light beam in the conventional example, respectively. 4 and 5 are sectional views of an embodiment of a light splitting prism according to the present invention, FIG. 5 is a view showing a method for forming an air layer in the above embodiment, and FIG. 6 is a view of the first prism of the above embodiment. It is a principal part expanded sectional view. 3 ... First prism, 3a ... Light splitting surface, 4 ... Second prism, 8 ... Air layer, 9 ... Spacer.

Claims (1)

[Claims] 1. A first prism having an incident surface and a light splitting surface inclined with respect to the incident surface and having a half mirror coat deposited thereon, and a first prism having a surface facing the light splitting surface with a minute air gap therebetween. It is composed of two prisms, and in the image forming light flux incident from the first prism, the component reflected by the light splitting surface is totally reflected by the incident surface of the first prism and guided to the focus detection element, A light splitting prism that allows a component that has passed through the light splitting surface to enter the photosensitive surface via the second prism, wherein among the light rays that are incident from the incident surface of the first prism to the outside of the photosensitive surface, When the component reflected by the light splitting surface is totally reflected by the incident surface of the first prism and the incident angle when entering the light splitting surface again is δ, the first condition is satisfied so that the following conditional expression is satisfied. Characterized by selecting a prism Light splitting prism. n sin δ ≧ n ₀ Here, n is the refractive index of the first prism, and n ₀ is the refractive index of air.