Quote Originally Posted by nrdavis


I can't help but be completely confused by this thread, and by the general frequency with which people on this forum multiply the maximum aperture of a lens by a 1.6x crop factor when it's used on a small sensor body. For instance saying the 17-55mm f/2.8 IS is effectively a 27-88mm f/4.5.


My confusion is this. Say I go outside on a sunny day, with an incident light meter, and get an exposure reading of ISO100 at 1/3200 second at f/2.8 And let's say I'm shooting with the three different bodies: an EOS 1V film camera, an EOS 5D mark II, and an EOS 50D. I'll say I'm using the 16-35mm f/2.8 L on the full frame bodies, and the 17-55mm f/2.8 IS on the 50D.


My best guess is that if I shoot a photo with each camera, lens wide open, at 1/3200 second shutter speed and ISO 100, all three would produce a correct (and identical) exposure on all three cameras, despite the variation in field of view, and depth-of-field. Thus, I would say for the purpose of exposure the lens is an f/2.8 lens regardless of film or sensor size.


So, what am I missing? Is the aperture adjustment only for depth-of-field calculations?
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Short answer: yes.


For most of us, in practical terms, the FOVCF has little to do with "correct" exposure. Daniel got into a very deep technical analysis that is concerned with total image quality--diffraction, noise, etc., not just simple shutter speed + aperture + ISO exposure determination.) Do remember that the camera takes care of a lot of this in its processing, e.g., in what "ISO 100" means in terms of the sensor output can vary from camera to camera.


I'll get back--have to leave right now.