For your future reference, perhaps this will help:
F-number - Wikipedia, the free encyclopedia
Essentially, the f/stop system is an idealistic approach that tries to
provide a reference number(f/stop) wherein the light transmission thru
a lens is a constant. In reality, there is light loss in a lens element
system that is not accounted for in the f/stop method because, usually,
the loss is insignificant. In lenses with many elements, such as a
cinematic zoom lens, this can be more than 1 f/stop.
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