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p.1 #6 · p.1 #6 · TStops vs FStops and Ev Difference: School Me  
Basically, Tstop is derived from direct measurement of light coming through the lens, while Fstop represents the potential amount of light that can enter the lens, based on geometry; i.e. aperture diameter (as seen through the entrance pupil) and focal length.
Fstop f/1.7 is onehalf stop faster than f/2 (i.e. onehalf EV).
Two fstop tables are attached, below. The first simply shows the sequence of fstop values for full stops, halfstops, and thirdstops, from f/1.0 to f/4.0. The second fstop table shows the relationship between 'n' (the power) and SQRT(2), on which the sequence of fstops is based. The squareroot (SQRT) of two is important, as that represents the change in aperture diameter when the aperture area is increased; doubling the area (onestop increase in EV) corresponds to increasing the diameter by SQRT(2) = 1.41...
Note that, for some obscure reason, not all fstop values are "roundedup" or "roundeddown" following normal practice. For example, we see "f/5.6", not "f/5.7", and "f/22", not "f/23". Maybe numbers ending with even numerals were considered to be more sociallyacceptable than those ending with odd numerals, which is odd.
© jcolwell 2017
Fstop values from f/1.0 to f/4.0, for full stops, halfstops, and thirdstops.
© jcolwell 2017
Fstop table for full stops, halfstops, and thirdstops.
