JPS6033377Y2 - Light receiving device that can change the angle of incidence - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a light receiving device used in an auto strobe.

As is well known, an auto strobe has a photometric circuit that uses a light receiving element that converts incident light into an electrical signal.This photometric circuit measures the brightness of the subject, and the light from the auto strobe measures the brightness of the subject. The auto strobe stops firing when the light intensity reaches the scheduled light level.

The angle of incidence on the light receiving element in this autostrobe is usually fixed.

However, in practical use, it is desirable to change the incident angle depending on the angle of illumination of the light from the autostrobe or depending on the angle of view of the lens of the camera using the autostrobe.

In other words, when using a wide-angle lens to increase the angle of irradiation from an auto strobe, the angle of incidence is increased so that light enters the photodetector from a wider range, and conversely, when using a telephoto lens, the angle of irradiation from the strobe is reduced. Sometimes it is desirable to reduce the angle of incidence.

It is easy to simply change the angle of incidence on the light receiving element.

However, if the amount of incident light varies accordingly, accurate photometry cannot be performed.

That is, the brightness on the surface of the light receiving element due to a subject having a constant brightness must remain constant even if the angle of incidence on the light receiving element is changed.

As a means to achieve this, for example,
As shown in Japanese Patent No. 78872, a zoom lens is known in which a zoom lens is placed in front of a light receiving element.

However, as is well known, zoom lenses are extremely expensive, sometimes costing more than the entire strobe, and cannot be used as a light receiving device for a strobe.

Therefore, an object of the present invention is to provide a light receiving device that has a simple structure and can keep the amount of light incident on the light receiving element constant for an object of the same brightness even if the angle of incidence on the light receiving element is changed. The purpose is to

Therefore, the present invention is characterized in that a hood with a length equal to the focal length of the lens is provided on the lens installed in front of the light receiving element so that the distance between the lens and the light receiving element can be changed. .

The brightness of the surface of the light receiving element is the illuminance of the image formed by the lens.

The illuminance E of the image produced by the lens system is determined by E = r?, where L is the brightness of the object and θ is half the angle at which the exit pupil of the lens system is viewed from the image point P, as shown in Fig. 2. It is expressed as rLsinθ.

D is the transmittance of the lens system. Therefore, if θ is the same, as long as the brightness of the object is the same, the illuminance E1, that is, the brightness of the surface of the light receiving element is constant.

If the distance between the lens and the light receiving element is changed, θ will also change, and the illuminance on the surface of the light receiving element will also change.

Therefore, simply placing a lens in front of the light receiving element and changing the distance from the light receiving element can change the angle of incidence on the light receiving element, but this will also change the illuminance on the surface of the light receiving element, so it is not possible to It cannot be photometered.

Next, a case where the hood 1 is installed on the lens 2 as shown in the embodiment shown in FIG. 1 will be described.

It is assumed that a light receiving element (not shown) is placed at point P.

Now, the length of hood 1 is 1, and the distance between lens 2 and point P is 8.
The height from the optical axis of the lens to hood 1 is h, and the angle of incidence is α
, the focal length of the lens 2 is f, and a ray of light incident on the lens 2 at an angle α is refracted by the lens, advances at an angle α', and reaches the focal plane F.

At this time, the distance from the optical axis of the lens at the position where the ray of light that passes through the leading edge of the hood 1 and enters the lens 2 intersects with the lens 2 is d, and the ray of light intersects with that surface from the center on the focal plane F. Letting the distance to the point be D, the following equation holds true.

tana = (h d) / 1 = D/ f
"" (1) tana = d/ s = D/ (f-
s) = (2) Therefore, d = fsh/ (fl-sl + sf), so by substituting this into equation (1), tana = h (f-s) / (fl-sl + sf)
...(3) a = tan-" (h (f s
) / (fl-sl+5f)) (4).

On the other hand, equation (2) is tana'=fh/ (S (f l) + fl)
”” (5).

It can be seen from equations (3) and (5) that if S is changed, α and α' will change.

Here, f=1, that is, the length of the hood is equal to the focal length of lens 2.

Then, equation (5) becomes tan a' = fh/f2= h/f
” (6).

h and f are constant. Therefore, α' remains unchanged.

On the other hand, equation (3) is tan a = h (f s ) /F
””(7).

Therefore, tana changes depending on Sl, that is, the distance between the lens and the light receiving element.

As described above, according to the present invention, if the length of the hood 1 is made equal to the focal length of the lens 2, the angle of incidence from the subject can be changed by changing the distance between the lens and the light receiving element. , the amount of light incident on the light receiving element can be made equal regardless of changes in the incident angle.

Moreover, since the present invention simply requires installing a hood with a length equal to the focal length of the lens, the structure can be extremely simple, and the cost can therefore be reduced.

[Brief explanation of the drawing]

FIG. 1 relates to an embodiment of the present invention, and is a schematic diagram showing the propagation of light rays, and FIG. 2 is an explanatory diagram of light rays in a general lens system. 1: Hood, 2: Lens.

Claims (1)

[Scope of utility model registration request] In a light-receiving device that consists of a light-receiving element and a lens provided in front of the light-receiving element, the angle of incidence on the light-receiving element is variable by changing the distance between the light-receiving element and the lens. A light receiving device characterized in that a hood having equal lengths is provided.