Mechanical focus maintaining apparatus

Claim 1

A FOCUS DETERMINATION APPARATUS FOR A PROJECTOR FOR SLIDES, EACH OF SAID SLIDES INCLUDING A FILM HELD ALONG ITS EDGES IN A MOUNT DEFINING THE IMAGE AREA TO BE PROJECTED, THE IMAGE AREA ESTABLISHING A FILM PLANE CAPABLE OF SHIFTING UPON "POPPING" OF SAID FILM DURING PROJECTION, THE INVENTION COMPRISING: A PROJECTION LENS HAVING AN AXIS ALONG WHICH THE IMAGE AREA WILL BE PROJECTED; SAID LENS HAVING A FOCAL PLANE LOCATED A FIXED DISTANCE BEHIND SAID LENS ALONG SAID PROJECTION AXIS WHEREIN A PROJECTED IMAGE IS IN SHARP FOCUS WHEN THE FILM PLANE IS COINCIDENT WITH SAID FOCAL PLANE; MEANS TO MOVE A SLIDE INTO ALIGNMENT WITH SAID PROJECTION AXIS; SUPPORT MEANS FOR RELATIVE MOVEMENT OF SAID PROJECTION LENS AND SAID ALIGNED SLIDE WHEREIN SAID FOCAL PLANE OF SAID PROJECTION LENS AND SAID FILM PLANE OF SAID SLIDE MAY BE MADE COINCIDENT; A FILM PLANE SENSING DEVICE CONNECTED TO SAID SUPPORT MEANS FOR MOVEMENT THEREWITH, SAID DEVICE ENGAGING A PORTION OF SAID IMAGE AREA OF SAID ALIGNED SLIDE; AND MEANS BIASING SAID SENSING DEVICE INTO ENGAGEMENT WITH SAID IMAGE AREA OF SAID ALIGNED SLIDE WHEREIN SAID FILM PLANE AND SAID FOCAL PLANE ARE MAINTAINED COINCIDENT WHEN SAID FILM "POPS" AND SAID SENSING DEVICE MOVES WITH SAID IMAGE AREA.