GB2226145A - Photometer device for a reflex camera - Google Patents

Description

Title of the Invention

Photometer device for a reflex camera This invention relates to a photometer device for a reflex camera comprising reflex view finder and imaging lens disposed between the pentaprism and the eyepiece outside the optical axis of the ima.aiing beam path and comprising a structured light receiver.

Many solutions are kno-..,vn for optical arrangements of selectively evaluating photometer devices for reflex cameras. Preferred locations for the arrangement of the light receiver are the space CD underneath the foldins, mirror, at the image-erecting surface or behind the light exit surface of the pentaprism. A distinction is made, in the measuring beam path, between arrangements having a single-surface light receiver combined with moving optic/mechanical cD components, and arrangements having a multi-surface light receiver combined with fixed optic/mechanical components. The last-named group has achieved the greater popularity because of its simpler construction and versatility in use. Systems of high local selectivity, such as are necessary for multi-field measuring devices, are preferably so constructed that the object field is imaged onto a multi-surface light receiver by the stationary imaging optic

1 behind the light exit surface of the pentaprism. These systems, according to the evaluation arrangement of the signal processing, make it possible to find the exposure value in the evaluation spot, part integral or multi- field according to a predetermined weighting cl of the actual light vaClues of the part surf aces. P or achieving a high efficiency of signal recovery, a rotationally asymmetrical refractive power distribution is stated for the scanning field lens of the view finder system according to OS 28 56 542, for example.

By use of the invention, a partial light evaluation in ref lex cameras which is correct in practice, is made po ssible.

An object of the present inventioln is the creation of a phot- ometer device for reflex cameras, by which a locally selective measurement of the brightness of the object is carried out, taking account of the optirium position of the centre of gravity of the sensitivity;,.vith integral evaluation, and which shall be uncompli- C cated in construction and space-saving.

According to the present invention there is provided a photometer device for a reflex camera, the reflex view finder of which possesses a scanning field lens lying in the intermediate image plane and a pentaprism, and wherein, between pentaprism and eyepiece, an imaging lens and a structured light receiver are disposed outside the optical axis of the imaging beam path, characterized in that, as viewed in the direction of the light, there is disposed before the imaging lens an

3 aperture stop, having a trapezium-shaped opening and making possible, by reason cf the geometry of its opening, an asymmetrical area utilization of the imaging lenz and, behind the imaging lens, there is disposed a shade stop in the region below the optical axis.

The imaging lens is preferably formed as a Fresnel plate with st-ructure applied on one side, on the side to.vards the light exit surface. Purthermore, the shade stop or diaphragm is disposed with its upper edge in the beam path so that the lo-aer half of the light receiver, as seen from the lo.-,e-. edge of the aperture stop, appears shadowed. As vie.7ed from the lower edge of the aperture stop, the part of the area of the liEht receiver that appears shadowed amounts to 0.5 to 1, while the part of the area of the aperture stop which appears shadowed as vieaed from the lower edge of the light receiver surface amounts to 0.3 to 0.6. one embodiment of the aperture stop according to this invention provides that, for its non-parallel lateral boundaries, the prism edges present in the upper part of the light exit surface can. themselves be used. lditll'i advantage, the shade stop or diaphragri may be constructed as a component of the surface of the casting compound of the light receiver. The photometer device accoraing to this invention is explained in greater detail below by reference to an example of embodiment thereof. The Figures in the associated drawing sho-a:

Fig. 1 a schematic illustration of a reflex view finder assembly -aith photometer device and Fig. 2 a perspective view of the photometer device.

In Fig. 1, reference 1 denotes the folding mirror of the reflex view finder assembly. The ima.ge beam path is deflected by this mirror to 1 the scanning field lens 2. The view finder image produced here is presented to the viewer by means of the eyepiece 4 through the pentaprism 3 in the usual form of the reflex view finder. Above the optical axis of the viewing beam path, an imaging lens 6 is situated near the light exit aiirface 31 of the pentaprism 3. This lens images directly onto the photosensitive surfaces 71/72 of the light receiver 7 the view finder image produced on the scanning field lens 2. With advantage, the imaging lens 6 is constructed as a Fresnel plate and has its Fresnel structure on the side towards the pentaprism 3.

Between the light exit surface 31 and the imaging lens 6, according to this invention a thin-walled aperture stop or diaphragm 5 is disposed. This possesses a trapezium-shaped opening for the optimum, utilization of the incident light and for promoting the object field evaluation, to be described later. Referred to a horizontal centre line, the aperture stop 5 has asymmetrical partial areas 51 and 52, the lower partial area 52 being larger than the upper partial area 51. Behind the imaging lens 6 is a shade stop or diaphragm 8, which is situated in the beam path in the region below the optical a;is. It is disposed with its upper edge 81 in the beam path in such manner that a half-shado-.u, decreasing from the bottom upwards, is produced on the light receiver surface. The proportion of the area of the light receiver 7 -.,,ihich appears to be shadowed, as viewed from the lower edge of the aperture stop 5, lies between 0.5 and 1, and the proportion of the area of the aperture stop 5 which appears to be shaded, as viewed from the lower edge of the light receiver surface, lies between 0.3 and 0.6. As a result of height reversal in the

1 1 imaging of the view finder image by the imaging lens 6, therefore, the light coining from the upper part of the object space is continuously, decreasingly attenuated compared with that from the lower part.

The same arrangement without a shade stop 8 results in the undesired overevaluation of the upper part of the object space, which ls typical for this geometry. By the special construction illustrated of the shade stop 8, assuming that the angle of incidence u of the light of the photometer device is given by u = 0 and taking into account the scattering property of the scanning field lens 2, a practice-correct, locally integral evaluation of the object field, having its centre of gravity below the middle of the field, is possible, without needing to carry out a stepped calibration in electrical channels of the receiver surfaces 72.

Furthermore, the photometer device is so constructed that the central partial area 71 of the light receiver 7 has a size conjugate with the central field 21 of the scanning field lens 2, corresponding to the imaging scale of the imaging lens 6. The central field 21 here occupies approximately 10 1& of the entire area of the view finder field and its edge is identical with the focus indicator outline disposed centrally in the view finder. Thus, in an evaluation of the signal which results solely from the partial area 71, a spotorientated, selective measurement is obtained.

It lies within the scope of this invention that, where the aperture stop or diaphragm is used, the prism edges present in the upper part 1 of the light exit surface can themselves act as the non-parallel lateral boundaries. Moreover, the shade stop may be constructed as a component of the surface of the casting compound of the light receiver.

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Photometer device for a reflex camera, the reflex view finder of which possesses a scanning field lens lying in the intermediate image plane and a pentaprism, and wherein, between pentaprism and eyepiece, an imaging lens and a structured light receiver are disposed outside the optical axis of the imaging beam path, characterized in that, as vie,,ted in the direction of the light, there is disposed before the imaging lens (6) an aperture stop (5), having a trapezium-shaped opening and making possible, by reason of the geometry of its opening, an asymmetrical area utilization of the imaging lens (6) and, behind the imaging lens (6), there is disposed a shade stop (8) in the region below the optical axis.

Claims (1)

1. Photometer device according to Claim 1, characterized in that the imaging

   lens (6) is constructed as a Fresnel plate with structure applied on the one side facing towards the light exit surface (31).

   Photometer device according to Claim 1, characterized in that the shade stop (8) lies with its upper edge (81) in the beam path in such a manner that, as seen from the lower edge of the aperture stop (5), the lower half of the light receiver (7) appears shadowed.

   Photometer device according to Claim 3, characterized in that the proportion of the area of the light receiver (7) that 1 appears shadowed as viewed from the lower edge of the aperture stop (5) lies between 0.5 and 1 and the proportion of the area of the aperture stop (5) that appears shadowed as viewed from the lower edge of the light receiver surface lies bet.,,pteen M 0.3 and 0.6...

   5.

   7 Photometir device according to Claim 1, characterized in that the shade stop (8) is constructed as a component of the surface of the casting compound of the light receiver (7).

   Photometer device according to Claim 1. characterized in that the prism edges present in the upper part of the light exit surface (31) are themselves provided as hon-parallel lateral boundaries to the aperture stop (5).

   Photaneter substantially as described with reference to the accanpanying drawing.

   Published 1990 atThe patent Office, State House, 86171 High HolboM.LondonWC1R4TP. Further copies may be-obtainedfrom The Patent Office. Sales Branch, St Mary Cray. Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mal"i Cray, Kent, Con. 1187