Abstract: In order to establish a predetermined position of focusing or focal-length adjustment for an optical objective, a mechanism for the axial shifting of one or more lens members has a lost-motion zone in its operating range in which a continuing displacement of a control element does not change the focal length and/or distance of the objective. The halting of the lens shift (or, possibly, a compensatory displacement of an otherwise stationary lens member) can be brought about mechanically, with the aid of suitably shaped cam tracks, or electromechanically, via analog or binary position sensors. Arrival of the adjusting mechanism in the lost-motion zone may be signaled to the user by a detent, by a change in frictional resistance, or by a visual or acoustical indicator.

Abstract: A film strip is continuously advanced past a film gate of a camera or a projector while light coming from the strip through the film gate is reflected by a pivotable mirror keeping a beam of light trained upon a specific area of the strip, i.e., a frame, during a major part of a recurrent operating cycle. The mirror is reciprocated during each operating cycle under the control of an electrical signal generator synchronized with the strip-feeding mechanism, either directly or through markings (e.g., perforations) on the film strip, and performs a relatively slow forward sweep followed by a relatively fast return sweep, the latter being effected by a pulse of one polarity immediately followed by a pulse of opposite polarity arresting the mirror drive. In one embodiment (FIG.