Why can a 400mm f/4 lens have a smaller-looking aperture than 100mm?
Asked 3/22/2019
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A 400mm f/4 lens should have an aperture of 100mm based on focal length divided by f-number. But when I look at these lenses, the iris mechanism inside doesn’t seem large enough to be 100mm across. How does that work? Is the effective aperture really that large, or do lens elements make a smaller physical opening behave like a larger one?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Your observations of the lens leads you to both a correct, and incorrect, conclusion.
Correct: the aperture (i.e., mechanical iris) of the lens is substantially smaller than the 10 cm it supposedly should be. Only the front element is anywhere near 10 cm diameter. Where the iris mechanism is in the lens barrel, the diameter is substantially smaller than 10 cm.
Incorrect: the stated aperture size is not actually 10 cm in diameter. In reality, it is the the apparent size of the wide-open aperture (the iris), when viewed through the front of the lens, that is 10 cm in diameter.
For any lens, the aperture when viewed through the front of the lens is known as the entrance pupil. It is the entrance pupil that is 10 cm in diameter.
See also:
Originally by user11924. Source · Licensed CC BY-SA 4.0
user11924
7y ago
0
Generated from our catalog & community — verify before relying on it.
The f-number is based on the diameter of the lens’s entrance pupil, not necessarily the physical diameter of the iris blades themselves.
For a 400mm f/4 lens, the required aperture is 100mm, but that 100mm is the apparent size of the aperture as seen through the front of the lens. Lens elements in front of the iris can magnify that opening, so the actual mechanical iris can be smaller than 100mm while appearing larger from the front.
So your observation is partly right: the iris inside the lens often is smaller than the number from the simple formula suggests. But the formula still holds, because it uses the entrance pupil diameter, not the bare physical opening.
This is also why zoom lenses can maintain a given f-number even though the physical iris may not change size much: optical groups alter the apparent size of the aperture.
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