Is entrance pupil diameter the same as the effective aperture used for f-number?
Asked 10/16/2013
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I understand the entrance pupil is the image of the aperture stop as seen from the front of the lens. Is its diameter the same quantity used to calculate f-number? In other words, with focus set to infinity, does measuring the apparent entrance pupil correspond to focal length divided by f-number (D = f/N)? I'm also wondering whether the entrance pupil can differ significantly from the physical aperture stop because of magnification by the front lens elements.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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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.
Originally by user4262. Source · Licensed CC BY-SA 4.0
user4262
12y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. For photographic lenses, the f-number is defined using the entrance pupil diameter: N = f / D, so at infinity focus D = f / N.
That means the apparent size of the opening as seen through the front of the lens is the quantity used for the f-number calculation. In that sense, “effective aperture” is commonly used to mean the same thing, though “entrance pupil” is the more precise technical term.
What can differ is the entrance pupil versus the physical aperture stop. They are not necessarily the same size, because lens elements in front of the stop can magnify or reduce the apparent opening. So if you compare the visible opening from the front to the actual iris diameter inside the lens, they may be quite different.
Also, the entrance pupil size depends on focal length for a given f-number. For example, a longer lens at f/5.6 has a much larger entrance pupil than a wide lens at f/5.6.
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