EXCERPT page containing first few paragraphs. 2017-04-29 00:18:44
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F-stop in effect is a measure of light gathering ability that is independent of the format size.
That’s because it takes both focal length and format size into account via a ratio of focal length to entrance pupil diameter:
ƒ-stop = (actual focal length) / (effective entrance pupil diameter)
The neat little trick involved is the ratio: for any particular format size the focal length and entrance pupil size will differ, but the same ratio (f-stop) means the same exposure, whether it’s a phone camera with a tiny sensor or a huge 11 X 14 inch view camera.
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