Any elementary book on photography explains that aperture values are
expressed in terms of the diameter of the opening as a fraction of the
focal length, so for a 100mm lens, f/4 indicates a diameter of 25mm. OK
so far, but how does this work at wide apertures on long lenses? I
can't measure a line longer than about 50mm across the throat of a K
mount, and yet I could fit to it a lens of 200mm with a maximum aperture
of f/2.8, which would require a diameter of nearly 71mm.
What I don't understand, Inspector, is how a 71mm-wide disc of light can
pass through a 50mm-wide hole. There are those on the List who
evidently understand optics beyond the elementary level, so can anyone
please explain?
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