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Author Topic: Compound lens characteristics - is there a standard? George May Film Handler Posts: 60 From: Bath, United Kingdom Registered: Nov 2001 posted 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  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 03-03-2008 09:24 AM                     ANSI/SMPTE 243M is the standard you are looking for the rules in the USA. Steve  |  IP: Logged

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