Film-Tech Forum ARCHIVE: Compound lens characteristics

This is topic Compound lens characteristics - is there a standard? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by George May (Member # 1060) on 03-03-2008, 07:44 AM:

I'm keen to use some conventional spherical backing lenses for projecting from other light sources than 35 mm projectors. I've been doing some experiments with various lenses of various focal lengths and with both 62.5 mm and 70.6 mm barrels.

Simple secondary school optics would lead you to expect that for a simple (one-element) lens projecting with a throw >> its focal length, the lens should be placed at a distance almost equal to its focal length from the gate/light source/object to focus an image at infinity. So far, so good.

Projection lenses are compound, multi-element, arrays, so you might expect the object-lens distance to vary from this simple model.

However I'm finding that there seems to be a large variation in the correct distance from the object to the back of the lens assembly; it's not simply roughly proportional to the focal length of the lens. It seems to vary by manufacturer, lens model, aperture, ...

The question I'm (slowly) getting to is this. Since projector head manufacturers don't know what lenses their equipment will be used with, where do they position the lens locating mount w.r.t. the gate? Is there a SMPTE or other standard that defines what the gate /lens assembly distance should be?

Can anyone help?

Thanks

George

Posted by Steve Guttag (Member # 268) on 03-03-2008, 09:24 AM:

ANSI/SMPTE 243M is the standard you are looking for the rules in the USA.

Steve