I understand that the entrance pupil is the image of the aperture as seen from the front of the lens. However, is the size of the entrance pupil the same as the size of the effective aperture? In other words, if I measure the size of the entrance pupil by looking at it, will that be equivalent to the focal length of the lens divided by the f-number (with the focus set to infinity)? I know that sometimes this is hard to estimate because of the shape of the aperture blades, but I was wondering whether pupil and aperture can be quite different (perhaps at very low magnification).
Answer
if I measure the size of the entrance pupil by looking at it, will that be equivalent to the focal length of the lens divided by the f-number
Yes. F-number is defined in terms of focal length and entrance pupil diameter, so if you know any two of: entrance pupil diameter, f-number, and focal length, you can calculate the third number.
The equation that Wikipedia gives is:
N = f/D
where N is f-number, f is focal length, and D is entrance pupil diameter. Dividing both sides by f and taking the reciprocal of both sides gives you:
D = f/N
Note this sentence from the linked article:
The entrance pupil diameter is not necessarily equal to the aperture stop diameter, because of the magnifying effect of lens elements in front of the aperture.
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