The present invention relates to an aperture control mechanism for a single lens reflex camera of lens interchangeable type in which the aperture control is achieved within a predetermined operable range (R) of a diaphragm control member and particularly to such aperture control mechanism utilizing a microcomputer for digital control so as to improve precision, resolution and reproductivity of the aperture control.
The aperture control mechanisms of the prior art have been of the so-called regular interval position control type in which the diaphragm control member has its stroke set in regular intervals with respect to adjacent aperture values of each objective. With the mechanism of such regular interval position control type, however, the stroke (ST) per 1 EV of the diaphragm control member is determined without any variation, based on the operable range (R) of the diaphragm control member and the range of the apex representing values (.DELTA.A.sub.v) corresponding to the aperture adjustable range of a particular objective belonging to a group of interchangeable lenses (hereinafter referred to simply as "aperture adjustable range"). For example, an objective for which the maximum aperture is F1.4 and the minimum aperture is F22, has its aperture adjustable range (.DELTA.A.sub.v) of 8 EV and said stroke (ST) per 1 EV of ST=R/8. In general, so far as the aperture control mechanism is concerned, the larger the stroke (ST) per 1 EV is, the higher the resolution and the precision achieved and the smaller the influence of the other factors such as mechanical play becomes. Usually, photographic cameras of lens interchangeable type utilize an interchangeable lens group typically including an objective having a maximum aperture of F2.8 and a minimum aperture of F22 and an objective having a maximum aperture of F4 and a minimum aperture of F32. The aperture adjustable range (.DELTA.A.sub.v) of such interchangeable objective is 6 EV and for regular interval position control will limit the operable range of the diaphragm control member to 6R/8. Thus, the remainder of 2R/8 will be a useless portion having no effect upon the aperture control, so far as the interchangeable objective having its aperture adjustable range of 6 EV is used.
It is necessary for a correct aperture control to stop down the objective accurately to an aperture value required by the actual condition of an objective to be photographed. However, the usual mechanical control is necessarily accompanied with a time lag from the moment at which the signal is received to the moment at which the mechanism is actually activated in accordance with this signal. Concerning the aperture control member included in the aperture control mechanism according to the prior art, if the signal with which the operation of stopping down is arrested is applied to said diaphragm control member just at the moment at which the diaphragm has been stopped down to the desired aperture value, the diaphragm would be actually stopped down to a position beyond the desired aperture value. To avoid such excessive stopping down, it has been found that said time lag must be compensated by a suitable measure, for example, by slightly earlier generation of the stop signal.