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What are F-stop and T-stop?

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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f-stop scales whole stops, 1/2 stops, 1/3 stops
             
Diaphragm as seen at optical center
Actual photos of Zeiss 100mm f/2 Makro-Planar diaphragm— f/2 through f/22
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