Exposure Compensation Information Forming Apparatus for a Camera

1. Field of the Invention

The present invention relates to the improvement of the light measurement compensation apparatus for the single lens reflex camera.

2. Description of the Prior Art

Generally, in the case of the so-called TTL totally opened light measuring system in accordance with which the light beam having passed through the photographing lens is directed to the light sensing element so as to measure the light amount, the object brightness BV can be calculated from the totally opened aperture compensation value AVo (F No. of the lens) and the aperture compensation value AVc and brightness BVo measured on the surface of the light sensing element as follows:

Until now, there are two systems for calculating the object brightness from the above relation, namely, the one in accordance with which the totally opened aperture compensation value AVo and the aperture compensation value AVc are obtained independently and the other in accordance with which the aperture compensation value AVc is written in the ROM and so on. This is done while the totally opened compensation value AVo is read out from the totally opened aperture compensation signal pin of the photographing lens so as to read out the aperture compensation value AVc corresponding thereto.

In the case of one of the conventional two systems, in accordance with which the aperture compensation value AVc and the totally opened aperture compensation value AVo are obtained independently from each other, for one signal pin of the lens two variable resistances, namely, the one for the aperture compensation and the other for the totally opened aperture compensation are mechanically engaged. Thus, it is necessary to provide two systems for the engaging mechanism, which necessarily becomes complicated, while it is also necessary to carry out the adjustment in such a manner that the two systems for the engaging mechanism assume a certain determined relation with each other. This causes the decrease of fidelity by adjustment and the increase of manufacturing cost, which is disadvantageous. Further, the case of the system in which the second ROM and so on are used, the calculation circuit becomes complicated and, therefore, it is difficult to cut the manufacturing cost, which is disadvantageous.

An object of the present invention is to provide an information forming apparatus for a camera, so designed that a first circuit for providing a voltage corresponding to the totally opened aperture compensation value AVo and a limiter circuit for transmitting only the output lower than a certain determined level, of the above circuit, are provided. Accordingly, AVo is obtained from the first circuit, while AVc information voltage is obtained from the limiter circuit as an output in such a manner that AVo and AVc informations are obtained by means of a very simple construction.

Further objects of the present invention will be obvious from the explanations to be made below in accordance with the drawings of an embodiment.

FIG. 1 shows an embodiment of the light measurement compensation circuit of the light measurement compensation apparatus in accordance with the present invention, for a camera.

FIG. 2(a) shows the characteristic curve for the totally opened aperture compensation information voltage VAVo of the circuit shown in FIG. 1.

FIG. 2(b) shows the characteristic curve for the aperture compensation information voltage VAVc of the limiter circuit shown in FIG. 1.

FIG. 3 shows an embodiment of the circuit of a camera to which the present invention applies.

Below, the present invention will be explained in accordance with an embodiment. The present invention is based upon the approximation between the aperture compensation value AVc and the totally opened aperture compensation value AVo, namely

whereby

K:proportion constant,

C:constant.

FIG. 1 shows an embodiment of the light measurement compensation apparatus in accordance with the present invention for the single lens reflex camera. In the drawing, element 1 is the information voltage producing circuit for producing the totally opened aperture compensation voltage VAVo corresponding to the totally opened aperture compensation value AVo and element 2 is the limiter circuit, which is supplied with the totally opened aperture compensation information voltage VAVo so as to produce the aperture compensation information voltage VAVc not higher than the standard voltage Vc. In the information voltage producing circuit 1, A1 is the operational amplifier, to whose non-inverting input terminal a constant voltage Vc is applied. RAVo is the totally opened aperture compensation information variable resistance whose value is adjusted in accordance with the projection amount of the totally opened aperture compensation pin of the photographing lens, which projection amount is different in accordance with the F value of the photographing lens.

The slide element of the variable resistance RAVo, to which a constant voltage KVc is applied through the resistance R1, is grounded. The constant voltage KVc is set higher than the constant voltage Vc. The connecting point of the resistance R1 with the variable resistance RAVo is connected to the inverting input of the operational amplifier A1. Rf is the feed back resistance connected between the inverting input of the operational amplifier A1 and the output, whereby the output of the operational amplifier A1 is applied as the totally opened aperture compensation information voltage VAVo to the operation circuit of the camera (not shown in the drawing) and to the limiter circuit 2, in which circuit A2 is the operational amplifier, to whose non-inverting input the totally opened aperture compensation information voltage VAVo is applied from the operational amplifier A1. D is the diode to whose cathode the output of the operational amplifier A2 is connected and to whose anode the constant voltage Vc is applied through the resistance R2. The connecting point of the diode D with the resistance R2 is connected to the non-inversing input of the operational amplifier A2, whereby the potential of the connecting point is applied as the aperture compensation information voltage VAVc to the operation circuit of the camera not shown in the drawing. The relation between VAVc and AVc is VAVc=f(-AVc).

Below, the operation of the above composition will be explained.

The value of the variable resistance RAVc is set in accordance with the F value of the photographing lens mounted on the camera, so as to decide the output voltage VAVo of the operational amplifier A1, whereby the smaller the F value of the photographing lens, the larger the value of the variable resistance RAVc is. Thus, the smaller the F value of the photographing lens is, the lower the output voltage VAVo of the operational amplifier A1 is, as is shown in FIG. 2(a) in such a manner when the Apex value of the F value is 3, the output voltage of the operational amplifier A1 is Vc. The output voltage of the operational amplifier A1 is applied as the totally opened aperture compensation information voltage VAVc to the operation circuit of the camera not shown in the drawing and to the non-inverting input of the operational amplifier A2. Because the operational amplifier A2 constitutes the limiter circuit 2 together with the diode D the output voltage of the operational amplifier A1, which is lower than the standard voltage Vc is taken out, so that the potential at the connecting point of the resistance R2 with the diode D in accordance with the output voltage of the operational amplifier A2 corresponding to the change of the F value of the photographing lens is as is shown in FIG. 2(b), when the F value of the photographing lens is larger than AVo=3, the voltage becomes constant (Vc). The potential at the connecting point of the resistance R2 with the diode D is applied as the aperture compensation information voltage VAVc to the operation circuit of the camera (not shown in the drawing).

Because, as is mentioned above, the relation between VAVc and AVc is negative, when the value AVo is smaller than 3 AVc=f(-AVo). Now let us suppose that when VAVc=Vc, Avc=0. Then, when AVo is larger than 3, AVc=0, while when AVo is smaller than 3, Avc=K(C-AVo). Thus, the above-mentioned approximation, AVc=K(C-AVo).gtoreq.0 is satisfied. Namely, the AVc information can be calculated from AVo very simply.

FIG. 3 shows an embodiment of the light measuring circuit applied to the compensation apparatus shown in FIG. 1. In the drawing, elements 1 and 2 are the compensation apparatus shown in FIG. 1. OP1 is the operational amplifier, between whose inputs the light sensing element SPC for TTL totally opened light measurement is connected and in whose feed back circuit the logarithmically compressing diode ZD is inserted. OP2 and OP3 are the calculating operational amplifier, Rsv is the variable resistance to which the film sensitivity information -Sv is set and M is the display meter.

Below, the operation of the circuit shown in FIG. 3 will be explained.

Because the light sensing element SPC is for the TTL totally opened light measurement, the operational amplifier OP1 produces the output BVo (=Bv-AVo-AVc), (Bv:brightness value, BVo:TTL totally opened light measurement output). Further, because the circuit 1 produces the output VAVo corresponding to AVo, while VAVo(=AVo) and BVo are applied to the inverting input of the operational amplifier OP1, which produces the output -BVo-AVo (=-Bv+AVo+AVc-AVo=-Bv+AVc) to be transmitted to the inverting input terminal of the operational amplifier OP2. Because, on the other hand, the circuit 2 produces VAVc, while on the resistance R.sub.sv the film sensitivity information -So is set, to the inverting input of the operational amplifier OP3, VAVc and -Sv are applied beside -Bv+AVc. Thus, the operational amplifier OP3 produces Bv-AVc-VAVc+Sv. Further, the relation between VAVc and AVc is negative, so that the operational amplifier OP3 produces Bv-AVc+AVc+Sv=Bv+Sv(=Ev), so as to apply Ev (Exposure value) to the meter, which displays the Ev value.

As is explained above in detail, in accordance with the present invention, it is taken into consideration that there exists an approximation between AVc and AVo in such a manner that a circuit for producing the voltage corresponding to AVo and a limiter circuit for allowing the passage of the output of the circuit below a certain determined level Vc are provided, whereby the limiter circuit produces the voltage corresponding to AVc. Thus, as is different from the conventional apparatus, without making use of either the complicated mechanism or the logic circuit, a circuit with high fidelity can be obtained in a very easy way. This contributes much to the realization of a very effective light measurement compensation apparatus for the camera.