Optics: measuring and testing – Photometers
Reexamination Certificate
1998-11-27
2001-04-24
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Photometers
Reexamination Certificate
active
06222622
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photometering apparatus for a camera having an interchangeable focusing plate.
2. Description of the Related Art
In general, a single lens reflex camera is provided with a view finder in which light carrying an object image formed by a photographing lens is converged onto a focusing plate (focusing screen) which is located in a position optically equivalent to a film surface, whereby the object image formed on the focusing plate can be enlarged and viewed as an erect image through a magnifier and an image erection system (pentagonal prism). To detect the brightness of the object, a photometering device (photometering sensor) is provided in the vicinity of the magnifier; the object image formed on the focusing plate is re-formed on a light receiving surface of the photometering sensor to measure the object brightness in accordance with the output of the photometering sensor corresponding to the object.
There are known various focusing plates including those which are provided on the center portion thereof with a split image or a microprism, or provided on the entire surface thereof with a mat.
In a camera in which a replaceable screen on which a desired focusing plate can be optionally mounted, the state, or the quantity of the object image light rays transmitted through the focusing plate varies depending on the characteristics of the mounted focusing plate. Therefore, for different focusing plates, the state, or quantity of the object light incident upon the photometering sensor is different, thus resulting in an unreliable measurement of the object brightness. This is shown in
FIGS. 16 and 17
by way of example.
FIG. 16
shows an example in which, in a spot beam photometering operation, an exposure error caused by the different characteristics of the focusing plates varies as the minimum diaphragm value (open aperture) Avmin of the photographing lens changes.
FIG. 17
shows an example in which an exposure error caused by the different characteristics of the focusing plates varies as the position of the exit pupil changes.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a photometering apparatus for a camera having an interchangeable focusing plate, in which the measured object brightness can be corrected in accordance with the characteristics of the selected focusing plate.
To achieve the object mentioned above, according to the present invention, there is provided a photometering apparatus for a camera including a photometering sensor which receives object light transmitted through a focusing plate and measures the brightness of an object, and a focusing plate holding device which detachably holds the focusing plate including: a discrimination device for discriminating the type of focusing plate held by the focusing plate holding device; and a control device for selecting at least one attribute from a plurality of photometering algorithms and parameters corresponding to the focusing plate discriminated by the discriminating device. The selected attribute can be an algorithm, a parameter, or a combination of both.
Preferably, the control device corrects a photometering correction value corresponding to the focusing plate discriminated by the discriminating device.
Preferably, the control device selects a photometering mode corresponding to the focusing plate discriminated by the discriminating device.
Preferably, the control device modifies a split-photometering algorithm which corresponds to the focusing plate discriminated by the discriminating device.
Preferably, the focusing plate is provided with a discrimination member which is adapted to discriminate the type of focusing plate. The discriminating device detects the discrimination member to thereby discriminate the type of focusing plate.
Preferably, the type of focusing plate is either: a mat type, a split image type, a microprism type, a whole surface mat type, or a Fresnel lens type.
Preferably, the correction of the photometering correction value includes a correction of the uniform level of the measured brightness.
Preferably, the correction of the photometering correction value includes a correction of the output level, of the photometering sensor which can be varied in accordance with an open diaphragm value.
Preferably, the correction of the photometering correction value includes a correction of the output level of the photometering sensor in accordance with the position of an exit pupil.
Preferably, the selection of the photometering mode is carried out by switching between a split-photometering operation and a center-weighted photometering operation.
Preferably, the selection of the photometering mode is carried out by switching between a spot-beam photometering operation and a center-weighted photometering operation.
Preferably, the selection of the photometering mode is carried out by the modification of a calculation expression for a center-weighted photometering operation.
Preferably, the modification of the split-photometering algorithm is carried out by modifying the weighting value of the split photometering areas.
Preferably, the parameters corresponding to the focusing plate include: the photometering error level, the minimum diaphragm value, and the exit pupil.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 09-325014 (filed on Nov. 26, 1997) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 4977424 (1990-12-01), Mukai et al.
patent: 5012268 (1991-04-01), Hirai
patent: 5289225 (1994-02-01), Hirai
Hirai Isamu
Ohkura Tadahisa
Asahi Kogaku Kogyo Kabushiki Kaisha
Font Frank G.
Greenblum & Bernstein P.L.C.
Nguyen Tu T.
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