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You are right there are a lot of thing the f can stand for, I think it is just a simplified math function sign where you usually had the upper and lower thresholds so f over 32 as max under 3.5 as min to 5.6 as current (does that make sense?).
The word stop I believe I am certain comes from the first cameras where the apeture opening was "yanked" by turning a wheel with holes in the to stop them.
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