Sparky wrote: I've just been reading about how in microscopes the illumination condenser system and its position/focal length are considered very critical, but I guess there you want maximum edge contrast for seeing cell outlines etc- not really what we want here.
But, in many ways, the problems are the same except the results you want are a bit different. On a microscope slide, the specimen isn't just one layer with an image on it but, rather, many layers at different levels with micro-organisms floating around. To see all these layers at once and in focus as much as possible, a focused light source is more desirable because it will do to the micro-organisms what it does to the scratches on the back side of the film. A diffused light source on a microscope would pretty much limit what is in focus to just a very thin area. So what's bad for a microscope is good for copying film because film, also, has many layers. When copying 35mm film, it really isn't much of an issue but when copying 8mm film, the thickness of the film relative to the frame size is considerable. To be able to copy just the emulsion side of the 8mm frame and leave the base side as soft as possible is ideal. Diffused light helps in that regard while focused light makes it near impossible because you end up seeing both the base and the emulsion in focus together, just like on a microscope.
EDIT: One of the
other reasons that early optical printers used a focused light source is because it would allow
multiple layers to be in focus more easily. That's important when bi-packing mattes and counter mattes for effects or titling work, especially when they were not emulsion to emulsion, which is really required when using a diffused light source.
Roger
