Re: Aperture vs Focal Ratio to determine exposure time
f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. YouBut, f-ratio has some importance in physics.
For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to f-ratio? It gets smaller proportional to f-ratio.
So, a 200" f/3 has the same size Airy disk as a 2" f/3.
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