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Old Jan 26, 2005, 6:30 PM   #1
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Is, for example, F2.8 on a 35mm the same opening size as F2.8 on my A200? I have a night exposure calculator left over from my 35mm days that I want to use with the A200. It is a simple slide calculator that gives examples of scenes and suggested aperture and shutter duration. I didn't know if the exposures would match.
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Old Jan 26, 2005, 6:57 PM   #2
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YES. F2.8 is the same on any camera or lens.
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Old Jan 26, 2005, 9:46 PM   #3
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There are two answers:

Yes, your calculator will work the same for digital as for chemical cameras.

No, the opening size is not the same. Nor is the opening size the same for a 35mm as for an 8x10" view camera at the same f/stop. The f/stop is the ratio of the focal length to the diameter of the lens opening: larger format cameras need a longer focal length to cover the same field of view.

Don't worry about the second point. The f/stop is used as a measure of lens opening so you don't have to worry about the actual size of the opening. If you want to puzzle it out, spend enough time reading to understand why the exposure time decreases by a factor of two whenever the f/stop is increased by a factor of the square root of two. If you don't want to puzzle it out, not knowing won't hurt your ability to take good photos.
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