Quote:
Originally Posted by jaapv
The problem is, the focal length of a lens is fixed at the given value only at infinity and lengthens as it gets focussed closer, creating a smaller aperture of course. So in your formula f is the focal length at infinity and N the aperture at infinity?
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No, focal length is an invariant of the lens (by definition). What you mean is extension, which changes by focussing. In the formula the effect of focussing (changing the extension) is represented by
m, the aspect ratio. You can of course eliminate m and introduce the extension instead, giving another expression. But usually the ratio is more convenient for macro setups. Accordingly
N does not depend on the extension. It is simply the (dimensionless) value you choose by turning the diaphragm.