GB2253491A - Exposure control apparatus in single lens reflex camera - Google Patents

Abstract

An exposure control apparatus in a single lens reflex camera is disclosed. A focusing plate interchanging mechanism is provided in which different kinds (14A, 14B) of focusing plates 14 having different light diffusion properties are interchanged. A divided brightness measuring device 18 measures object brightnesses of the divided object brightness measuring areas which are set on the focusing plate 14 and a discriminating switch discriminates the kinds (14A, 14B) of focusing plates 14 which have been selected. An exposure factor calculator calculates exposure factors using selected outputs of the divided object brightness measuring areas, wherein the outputs are selected in accordance with the focusing plate 14 discriminated by the discriminator. <IMAGE>

Description

2253491 - 1 EXPOSURE CONTROL APPARATUS IN SINGLE LENS REFLEX CAMERA The

present invention relates to a single lens reflex camera having an interchangeable focusing plate and a divided brightness measuring device.

A divided brightness measuring device in which a picture plane (focusing plate) is divided into a plurality of brightness measuring areas is a known type of brightness measuring device in a single lens reflex camera. The brightness which is measured in each brightness measuring area is weighted with a specific value to determine an optimum exposure value. In such a known divided brightness measuring device (evaluation measurement system), a matt focusing plate having a uniform diffusion surface is used.

When the matt focusing plate. is used, it is difficult for a photographer to confirm the focal state by his or her naked e3e upon manual focusing. However, recent cameras having an automatic focusing function are free from this difficulty, and accordingly, the AF cameras usually employ the matt focusing plate. The diffusion surface of the matt focusing plate referred to includes any surface which diffuses light, such as sand-polished surfaces, fine lens composite - 2 surfaces, or fine prism surfaces.

On the other hand, in a known expensive camera having an interchangeable focusing plate, if a focusing plate (e.g., a center prism-provided focusing plate) having a split micro prism or a cross micro prism, usually provided on a center portion thereof and capable of being easily focused, is used, the following problem exists.

Namely, the center prism-provided focusing plate has no diffusibility at the center portion (prism portion) thereof. Consequently, little or no light reaches the center brightness measuring area of the divided brightness measuring device corresponding to the central prism portion of the focusing plate. In the evaluation measurement by the divided brightness measuring device, the divided brightness measuring areas to be measured are all weighted with specific weighting values to determine an optimum exposure value, regardless of the kind of focusing plate, as mentioned above Accordingly, if the divided brightness measurement is effected using the center prism-provided focusing plate, the evaluation measurement includes the measurement (weighted brightness value) of the central measuring area upon which little or no light is made incident. This makes it impossible to obtain an optimum or accurate exposure value. In particular, generally speaking, since the brightness of an object at the center - 3 portion of the picture plane is weighted with a higher weighting value, the exposure error caused becomes more serious.

Furthermore, a diffusive focusing plate having a partial light transmission portion usually provided at the center portion thereof is also known and is usually used in microphotography or astrophotography, etc. The transmission portion is provided with a cross line (groove). The diffusive focusing plate also has the same problem as mentioned above.

A focusing plate having a uniform diffusion surface will be referred to hereinafter as a "matt focusing plate", and one having a diffusion surface with a partial non-diffusive surface as a "non-diffusion surf aceprovided focusing plate", respectively.

An object of the present invention is to provide a single lens reflex camera having an interchangeable focusing plate and a divided brightness measuring device with an exposure control apparatus, wherein an optimum (accurate) exposure value can always be obtained even when a non-diffusive surface-provided focusing plate is used in the camera.

According to the present invention, there is provided an - 4 exposure control apparatus for a single lens ref lex camera having a divided brightness measuring device, the apparatus comprising a focusing plate interchanging mechanism in which different kinds of focusing plates having different light diffusion properties are interchangeable; a divided brightness measuring means for measuring the object brightnesses Of the divided object brightness measuring areas which are defined on said focusing plate; a discriminating means for discriminating the kind of focusing plate which has been selected; and an exposure factor calculating means for calculating exposure factors, wherein, the exposure factor calculating means selectively uses the outputs of the divided object brightness measuring areas in accordance with the focusing plate discriminated by the discriminating means.

According to another aspect of the present Invention, there is provided an exposure control apparatus for a single lens reflex camera, the apparatus comprising a focusing plate interchanging mechanism in which a matt focusing plate having a unifbrm diffusion surface and a non- diffusion portionprovided focusing plate having a partial non-diffusion surface portion can be interchanged, said matt focusing plate and said non-diffusion port Ion-provided focusing plate being provided with a plurality of divided object brightness measuring areas including a nondiffusion portion brightness measuring area corresponding to the non-diffusion portion of the nondiffusion port ion-provided focusing plate, a divided brightness measuring means for measuring object brightnesses of the divided object brightness measuring areas, a discriminating means for discriminating the matt focusing plates and the non-diffusion portion-provided focusing plate; and an exposure factor calculating means for calculating exposure factors in accordance with the measurements of the divided object brightness measuring areas including the non-diffusion portion brightness measuring area when the matt focusing plate is discriminated by the discriminating means, and in accordance with the measurements of the divided object brightness measuring areas other than the non-diffusion portion brightness measuring area when the non-diffusion portion-provided focusing plate is discriminated by the discriminating means.

With this arrangement, if the surface-provided focusing plate in which no incident upon a non-diffusive brightness measuringarea attached to the camera, the-measurement obtained from non-diffusive brightness measuring area is not used calculate the optimum exposure value, and accordingly, exposure error can be eliminated or minimized.

non-diffusive light is is the to an According to another aspect of the invention, there is provided an exposure control apparatus for a single lens reflex camera, the apparatus comprising; a focusing plate interchanging mechanism in which a matt focusing plate having a uniform diffusion surface and a nondiffusion portion-provided focusing plate having a partial nondiffusion surface portion can be interchanged; a divided brightness measuring means for measuring object brightnesses of a plurality of divided object brightness measuring areas which are defined on said matt focusing plate or said non-diffusion portion- provided focusing plate, said plurality of divided object brightness measuring areas including an area corresponding to said partial non- diffusion surface portion of said non-diffusion portion-provided focusing plate; a discriminating means for discriminating said matt focusing plate and said non-diffusion portion-provided focusing plate; and an exposure factor calculating means for calculating exposure factors in accordance with the measurements of said divided object brightness measuring areas including said area corresponding to said non-diffusion portion when said matt focusing plate is discriminated by said discriminating means, and in accordance with said measurements of said divided object brightness measuring areas other than said area corresponding to said non-diffusion portion when said non-diffusion portion- 1 - 7 provided focusing plate is discriminated by said discriminating means.

in a preferred embodiment, a non-diffusion surfaceprovided focusing plate is provided which has a split micro prism or a cross micro prism at the center portion thereof and a matt surf ace.surrounding said center split micro prism or said center cross micro prism.

Preferably, the non-diffusion surface-provided focusing plate has a transmission portion with a cross line at a center portion thereof and a matt surf ace surrounding said center transmission portion.

According to a further aspect of the invention, there is provided an exposure control apparatus for a single lens ref lex camera, the apparatus. comprising; a divided brightness measuring means for measuring object brightnesses of divided object brightness measuring areas formed on a focusing plate; and, an exposure factor calculating means for calculating exposure f actors by using selected outputs of said divided object brightness measuring areas, the outputs being selected by said divided brightness measuring means.

According to a further aspect of the invention, there is provided an exposure control apparatus for a single lens ref lex camera, comprising; a - a focusing plate interchanging mechanism in which different kinds of focusing plates having different light diffusion properties are interchangeable; and, a discriminating means f or discriminating the kind of focusing plate which has been selected.

- 9 An example of the invention will be described below in detail with reference to the accompanying drawings, in which:- Figure I is a longitudinal sectional view of a single lens reflex camera having an exposure control apparatus embodying the present invention; Figure 2A is a schematic perspective view of a focusing plate having a prism at a center portion thereof (central prism-provided focusing plate); Figure 2B is a schematic perspective view of a focusing plate having a central transmission portion with a cross line; Figure 2C is focusing plate; a schematic perspective view of a matt Figure 2D is a sectional view of the center portion of the focusing plate having a central transmission portion with a cross line, shown in Fig. 2B; Figure 3A is a front elevational view of a focusing plate discriminating switch when the central prism-provided focusing plate shown in Fig. 2A or the focusing plate having a central transmission portion with a cross line shown in Fig. 2B is attached; Figure 3B is a front elevational view of the focusing plate discriminating switch when the matt focusing plate - 10 shown in Fig. 2C is attached; Figure 4 is a plan view of an example of an evaluation measurement element of a divided brightness measuring device; Figure 5 is a diagram showing the intensity distribution of transmission light when light is made incident upon the central prism-provided focusing-plate shown in Fig. 2A; Figure 6 is a block diagram of the exposure control apparatus embodying the present invention; and, Figure 7 is a f low chart of a control operation of the exposure control apparatus embodying the present invention.

A single lens reflex camera as shown in Figure 1 has a taking lens 11 through which light (a bundle of rays from an object to be taken) is transmitted. The light transmitted through the taking lens 11 is reflected by a mirror 12 to form an object image on a focusing plate 14. The focusing plate 14 and a film plane 13 are equidistant from the mirror 12, so that the object image can be observed through a pentagonal prism 15 and an ocular system (eyepiece) 16. A brightness measuring lens 17 is located behind the pentagonal prism 15 and above the eyepiece 16 in - 11 such a manner so as to project an object image, reduced in size, onto an evaluation measurement element 18. In this projection, the object image formed on the focusing plate 14 is used as a secondary light source.

The focusing plate 14 is supported at a predetermined position by a focusing plate holder 20 which is rotatable about the axis of a shaft 19 and which is disengageably engaged by a holder hook 21 per se known. As is well known, the interchangeable focusing plate 14 is detachably attached to the focusing plate holder 20. Figures 2A, 2B and 2C schematically show three kinds of interchangeable focusing plates, i.e., a center prism-provided focusing plate (a focusing plate having a prism at the center portion thereof) 14A, a center transmission port ion-provided focusing plate (a focusing plate having a transmission cross line at the center portion thereof) 14B, and a matt focusing plate 14C, respectively.

The center prism-provided focusing plate 14A is provided at the center thereof with a prism portion 22, such as a split micro prism or a cross micro prism, or the like. The remaining portion of the center prismprovided focusing plate 14A forms a light diffusion surface (matt surface) 23.

The center transmission portion-provided focusing plate 14B is provided on the center thereof with a transmission portion 24, as shown in Fig. 2D, and the remaining portion surrounding the center transmission portion 24 forms a light diffusion surface (matt surface) 23. The transmission portion 24 is provided thereon with a grooved cross line 25. The center transmission port ion-provided focusing plate 14B is in focus when there is no deviation of an image from the cross line 25 even if a photographer slightly moves his or her eyes in the left and right directions when viewing through the finder.

Conversely, when there is a deviation of the image from the cross line 25, the center transmission portion-provided focusing plate 14B will be out of focus.

As is well known, the prism portion 22 of the center prism-provided focusing plate 14A forms a clear image which can be viewed through the finder when the focusing plate 14A is in focus, but the image is dimmed due to the prism function of the prism portion 22 when the focusing plate 14A is out of focus.

Since the image is substantially dimmed by the prism portion 22 even due to a slight amount of defocus, the center prism-provided focusing plate 14A can be advantageously used for manual focusing (focus adjustment by a photographer's naked eye). Although the dimmed image can also be observed by means of the diffusion surface 23, the dimness is not as great as that caused by the prism - 13 portion 22.

Both the center prism-provided focusing plate 14A and the center transmission port ion-provided focusing plate 14B will be referred to hereinafter as a non-diffusion surface-provided focusing plate 14AB. The matt focusing plate 14C, on the other hand, has a whole diffusion surface 23. The diffusion surface is not limited to a specific surface and can be. made of any surface which substantially diffuses light, such as a sandpolished surface, a fine lens composite surface (a composite surface made of a large number of f ine lens surfaces), or a fine prism surface (a prism surface made of a large number of fine prism surfaces), etc.

Each of the non-diffusion surface-provided focusing plates 14AB and the matt focusing plate 14C has an engaging projection 26 on one side face thereof. The non-diffusion surface-provided focusing plate 14AB has a discriminating projection 27 in addition to the engaging projection 26.

The discriminating projection 27 turns a focusing plate discriminating switch 28 (Figs. 3A and 3B) OFF when the non-diffusion surface-provided focusing plate 14AB is mounted to the camera body. The focusing plate discriminating switch 28 has a pair of switch contacts 28a and 28b which are normally in contact with each other, thereby maintaining the discriminating switch 28 in the ON position. When the non-diffusion surfaceprovidedfocusing plate 14AB is mounted to the camera body at a predetermined position, the discriminating projection 27 thereof separates the switch contact 28a from the switch contact 28b, thereby turning the discriminating switch 28 OFF.

Conversely, when the matt focusing plate 14C which has no discriminating projection 27 is mounted to the camera body, the discriminating switch 28 remains in the ON position due to the absence of the discriminating projection 27.

Figure 4 shows an example of the divided brightness measuring areas of the evaluation measurement element 18. In the arrangement shown in Fig. 4, there are nine brightness measuring areas including one central brightness measuring area A, four inner peripheral brightness measuring areas Bl, B2, Cl, and C2 surrounding the central brightness measuring area, and four outer peripheral brightness measuring areas D1, D2, El, and E2 surrounding the inner peripheral brightness measuring areas Bl, B2, Cl, and C2. The central brightness measuring area A corresponds to the prism portion 22 (or the transmission portion 24) of the non-diffusion surface-provided focusing plate 14AB.

These divided brightness measuring areas of the evaluation measurement element 18 measure the brightness of the corresponding portions of the object image formed on - is - the non-diffusion surface-provided focusing plate 14AB or the matt focusing plate 14C. As far as the object image formed on the diffusion surface 23 is concerned, the brightness thereof can be precisely measured. However, since the prism portion 22 or the transmission portion 24 refracts light in a predetermined direction, little or no light reaches the central brightness measuring area A, as shown in Fig. 5.

In Fig. 5, it is assumed that the non-diffusion surf ace- provided focusing plate 14AB is scanned with point light in a lateral direction from L to R along a center line of the front surface of the focusing plate 14AB, and the quantity of light transmitted through the focusing plate 14AB is detected at the other surface (rear surface) thereof. As can be seen in Fig. 5, the quantity of light is less at the center portion corresponding to the prism portion 22 or the transmission portion 24.

This problem is solved by an exposure control apparatus 30 embodying the present invention, as shown in Fig. 6. In Fig. 6, the exposure control apparatus 30 of the present invention includes a brightness measuring element selecting (excluding) means 31, an evaluation measurement calculating means 32, and an exposure factor setting (determining) means 33. The output signals of the nine divided brightness measuring areas A through E2 and the - 16 output signal of the focusing (focusing plate discriminating brightness measuring element brightness measuring element the output of the the nondiffusion mounted to the camera body, measurement calculating measurement based on the plate discriminating switch means) 28 are input to the selecting means 31. The selecting means 31 excludes central brightness measuring area A when surface-provided focusing plate 14AB is so that the evaluation means 32 performs the evaluation outputs of the divided brightness measuring areas without the output of the central brightness measuring area A. Thereafter, the exposure factor setting means 33 determines optimum (correct) exposure factors (shutter speed, diaphragm value, etc.).

When the matt focusing plate 14C is mounted to the camera body, the exposure factors are similarly calculated based on all of the outputs of the nine divided brightness measuring areas.

Figure 7 shows a flow chart of the control operations mentioned above.

In Fig. 7, whether or not the focusing plate discriminating switch 28 is turned ON, namely whether the non-diffusion surface-provided focusing plate 14AB is mounted to the camera body is checked at step S 11. If the non-diffusion surface-provided focusing plate 14AB is mounted to the camera body, the measurement of the central brightness measuring area A is excluded that is, the measurements of the eight divided brightness measuring areas other than the central brightness measuring area A are selected at step S12. Thereafter, the exposure factor calculation Q is carried out based on the measurements of the eight divided brightness measuring areas other than the central brightness measuring area A (steps S12 and S13).

If the non-diffusion surface-provided focusing plate 14AB is not mounted to the camera body, namely if the presence of the matt focusing plate 14C mounted to the camera body is detected at step S11, the measurements of all of the nine divided brightness measuring areas are selected at step S14.

Thereafter, the exposure factor calculation P is carried out based on the measurements of all of the nine divided brightness measuring ares (step S15).

Thus, optimum (correct) exposure factors can be -calculated to obtain an optimum exposure value.

Although the above mentioned embodiment is applied to non-diffusion surf ace-provided focusing plate 14AB having non-diffusion portion at a center portion thereof, the present invention can be similarly applied to a non-diffusion surface-provided focusing plate having a non-diffusion portion at a portion thereof other than the center.

As can be understood from the above discussion, with the present invention,in a single lens reflex camera having a divided brightness measuring device and an interchangeable focusing plate, a correct exposure value can be always obtained regardless of the kind of focusing plate to be used.

Claims (11)

1. An exposure control apparatus f or a single lens reflex camera, said apparatus comprising:

a focusing plate interchanging mechanism in which various types of focusing plates having dif ferent light diffusion properties are interchangeable; a divided brightness measuring means for measuring object brightnesses of a plurality of divided object brightness measuring areas which are defined on a said focusing plate; a discriminating means f or discriminating said various types of focusing plates; and, an exposure factor calculating means for calculating exposure f actors by using selected outputs of said divided object brightness measuring areas, said outputs being selected in accordance with a said focusing plate discriminated by said discriminating means.

2. An exposure control apparatus according to claim 1, further comprising a focusing plate consisting of either a matt focusing plate having a substantially uniform diffusion surface or a non-dif fusion surf aceprovided focusing plate having a partial non-diffusion portion.

3. An exposure control apparatus according to claim 2, wherein said divided object brightness measuring areas defined on said focusing plate include a brightness measuring area - 20 corresponding to said partial non-dif fusion portion of said nondiffusion surface-provided focusing plate.

4. An exposure control apparatus according to claim 2 or 3, wherein said exposure factor calculating means calculates said exposure factors in accordance with outputs of all of said divided object brightness measuring areas when said matt focusing plate is discriminated by said discriminating means and in accordance with said outputs of some of said divided object brightness measuring areas when said non-dif fusion surf aceprovided focusing plate is discriminated by said discriminating means.

5. An exposure control apparatus according to claim 4, wherein said exposure factor calculating means calculates said exposure factors in accordance with said outputs of all of said divided object brightness measuring areas including said nondiffusion portion brightness measuring area portion when said matt focusing plate is discriminated by said discriminating means and in accordance with said outputs of said divided object brightness measuring areas other than said non-dif fusion portion b rightness measuring area portion when said non-diffusion surface- provided focusing plate is discriminated by said discriminating means.

6. An exposure control apparatus according to any one of claims 2 to 5, wherein said non-diffusion surface-provided focusing plate has a split micro prism or a cross micro prism - 21 at the center portion thereof and a matt surf ace surrounding said center split micro prism or said center cross micro prism.

7. An exposure control apparatus according to any one of claims 2 to 5, wherein said non-diffusion surface-provided focusing plate has a transmission portion with a cross line at said center portion thereof and a matt surf ace surrounding said center transmission portion.

S. An exposure control apparatus according to any one of the preceding claims, wherein said discriminating means comprises a discrimination switch which is turned ON and OFF by a discriminating projection provided on a specific focusing plate.

9. A single lens reflex camera including an exposure control apparatus according to any one of the preceding claims.

10. An exposure control apparatus substantially as hereinbefore described with reference to the accompanying drawings.

11. A single lens reflex camera substantially as hereinbefore described with reference to the accompanying drawings.