GB2036990A - Camera exposure control circuit - Google Patents

Abstract

Upon shutter release: (a) brightness measured at 2 with the diaphragm 1 fully open is stored at 5 to calculate shutter time (b) the diaphragm stops down to a manually pre-set value, but is halted before reaching that value if the detected brightness falls below a certain value (set in at 4). Preferably if the diaphragm does reach its pre-set value, the shutter time is automatically re-adjusted to take account of any difference between the actual and pre-set diaphragm values. If the diaphragm is halted before reaching its pre-set value, no adjustment of shutter time is made. In this manner the exposure setting errors arising in the case of lower illuminances on the photo-conductive cell 2 are eliminated and the measurement range of the camera is increased. <IMAGE>

Description

SPECIFICATION Circuit arrangement for the exposure setting of photographic cameras The invention relates to a circuit arrangement for exposure setting of photographic cameras wherein the luminance of the object is measured and the output signal is used for the automatic electronic exposure setting.

There are known exposure setting circuit arrangements wherein the luminance of the object is initially measured while the diaphragm is open and the stop size associated with a pre-selected exposure time is calculated. As the camera release is depressed further, the lens diaphragm is closed and the illuminance passing onto the photoconductive cell is reduced during this process.

When the calculated f/value is reached, a diaphragm stopping signal is emitted. On account of factors specific to cameras, the diaphragm stoppage is however not effected directly when the stopping signal appears. The diaphragm setting error resulting therefrom is to be compensated for in that a difference signal between the desired and actual values of the diaphragm setting, which signal appears after the darkening of the photo-conductive cell, is utilised for the minimal correction of the pre-selected exposure time (DE AS 2,256,602).

The effect intended by said working diaphragm measurement for the correction of the errors arising during the diaphragm setting is unintentionally cancelled by the greater errors arising during the measurement of the lesser illuminance on the photo-conductive cell in the range of low object luminances.

The invention aims at improving the illumination accuracy in the range of low object luminances.

The object of the invention is the provision of a circuit arrangement for setting the illumination of photographic cameras, with the aid of which the exposure setting errors arising in the case of lower illuminances on the photo-conductive cell are eliminated and the measurement range of the camera is increased.

According to the invention, this problem is solved in that use is made of an opto-electric level detector which is designed as an actuator responding to a pre-set minimal object luminance for the automatic interruption of a working diaphragm measurement which in principle follows an open diaphragm measurement.

Preferably, the interruption level of the level detector is adjustable and the object brightness signal fed to the level detector can be derived from the exposure measuring circuit of the camera.

The invention will hereinafter be explained in more detail with reference to an exemplified embodiment.

In the drawings FIGURE 1 shows a block circuit diagram, and FIGURE 2 shows a flow diagram.

As shown in Fig. 1 , an input amplifier 3 is provided downstream of a photo-conductive cell 2 which is arranged behind a lens diaphragm 1. This amplifier is connected directly, on the one hand, and through a level detector 4, on the other hand, to a circuit complex 5 which comprises a sequence control circuit determining the functional sequence of the exposure time setting as well as a computing and storage circuit.

Designated 6 is a control circuit for a diaphragm stopping magnet 7, a shutter control magnet 8 as well as an exposure time indicating device 9.

The mode of operation is as follows First, the sensitivity x of the film material used as well as the desired stop size k is electrically input into the circuit complex 5. The actuation of a camera release not shown causes at the point in time t, a switch, which is also not shown, to be closed and the entire camera circuitry to be applied to battery voltage through this switch as well as the circuit complex 5 (Fig. 2, a). By this means, the measurement of the object brightness has been initiated at the maximum opening of the lens diaphragm 1 (Fig. 2, b). The signal emitted by the photo-conductive cell 2 is amplified in the input amplifier 3 and is fed via the direct connection to the circuit complex 5. In the latter, the necessary exposure time is calculated and is dispiayed in the indicating device 9 through the control circuit 6.As the camera release is depressed further, the open diaphragm measurement is completed (t2) and the lens diaphragm closes until the closing movement is interrupted in known manner by the diaphragm stopping magnet 7 through the control circuit 6.

At this point in time t3, the diaphragm has reached the preselected stop value except for a deviation Ak, Fig. 2 c.

If the object luminance is sufficient, there now occurs in the-area t4, ts, in order to compensate for the diaphragm setting error A k, another object brightness measurement with the working diaphragm set, Fig. 2 d. As a result of this measurement, the exposure time pre-indicated in connection with the open diaphragm measurement is corrected by a corresponding amount in the circuit complex 5. With the corrected exposure time, the exposure is then initiated at t8 and is completed at t, by means of the shutter control magnet 8, Fig. 2 e.

If, in contrast to the afore-mentioned condition, the object illuminance is below a specific minimal value, the level detector responds and causes the sequence control circuit contained in the circuit complex 5 to interrupt the described working diaphragm measurement. In this case, the exposure time formation is effected only on the basis of the value calculated during the open diaphragm measurement. On account of the now uncorrected diaphragm setting error, there does indeed occur a faulty exposure, but this is by far less pronounced than it would be if a working diaphragm measurement were still to be carried out under these conditions of brightness.

For improving the accuracy, the control threshold of the level detector is varied by the number of the stop steps which have respectively been pre-selected or the appraisal of the level is effected directly after the diaphragm closure at a constant control threshold. Even when the object luminances are low, a camera provided with the device according to the invention thus ensures an acceptable exposure accuracy and thus a widened working range.

Claims (5)

1. A circuit arrangement for the exposure setting of photographic cameras wherein the measurement of the object luminance is carried out initially while the diaphragm is open and, following the automatic setting of the diaphragm to a manually pre-selected value, while the diaphragm is working and the output signal is fed to a computing and storage circuit for the exposure time formation, characterised in that use is made of a photo-electric level detector which is designed as an actuator responding to a pre-set minimal object luminance for the automatic interruption of the working diaphragm measurement which in principle follows the open diaphragm measurement.

2. A circuit arrangement as claimed in Claim 1, characterised in that the output signal from the level detector is fed to a sequence control circuit which is contained in a circuit complex and which can be changed over to exposure setting following the open diaphragm measurement and the subsequent working diaphragm measurement or to exposure setting following an open diaphragm measurement only.

3. A circuit arrangement as claimed in Claim 1, characterised in that the control threshold of the level detector is adjustable.

4. A circuit arrangement as claimed in Claim 1, characterised in that the object brightness signal fed to the level detector is derived from the exposure measuring circuit of the camera.

5. A circuit arrangement for the exposure setting of photographic cameras substantially as described with reference to the accompanying drawings.