Rca Tk-41

[old_tv_nut]

Color camera optics including the TK-41 and later could not possibly do operational focusing by moving the pickup tubes - there is just too much that has to be aligned and bolted down so that registration doesn't get disturbed. So, operational focus had to be via the front-end. On the TK-41 with multiple fixed lenses on a turret, I believe operational focussing was done by moving the whole lens drum in and out. I don't know what adjustment was available for back focus. Regarding the iris, the TK-41 had a motor controlled iris in the relay lens path, which IIRC limited the maximum opening to F/4. (I'm out of town this week, so can't refer to the literature.) My understanding of how the optics worked was that it didn't matter if you opened the objective lens beyond f/4, f/4 is all you got due to the relay lens - so I'd guess the objectives lenses would all be opened further than f/4 to make sure they weren't affecting the iris adjustment. Unless, of course, one of the lenses was smaller than f/4, in which case I guess you could do it two ways: set all of them to the same smaller aperture, if you wanted consistency, or set all to max aperture, if you wanted the max sensitivity each could give (up to f/4).