Can you estimate a lens’s maximum aperture by measuring its entrance pupil?
Asked 10/14/2013
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I have an unknown lens and don’t know its focal length or marked f-number. I’d like to estimate how much light it can gather. If I measure the apparent size of the entrance pupil (for example by looking into the lens at maximum aperture and comparing it to a ruler placed at the same apparent plane), can I use that to estimate the lens’s aperture or light-gathering ability? Or do I also need the focal length/field of view to make a meaningful calculation?
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 you don't know the focal length or the aperture, there is no way to figure it out. The amount of light that a lens is going to let in depends on the field of view of the lens as well as the size of the entrance. You don't know enough for the field of view, so you can't know how much light it gathers.
An alternative method that would work would be to make an area light source of a known size and place it a known distance from the lens. You could then measure the size of the circle project by the lens. This would allow you to determine the focal length and field of view, then you can perform the rest of your calculation to determine the speed of the lens.
Originally by user11392. Source · Licensed CC BY-SA 4.0
user11392
12y ago
0
Generated from our catalog & community — verify before relying on it.
Measuring the entrance pupil can give only a rough estimate, and by itself it is not enough to determine total light-gathering ability.
For a lens, the usual specification is f-number:
f-number = focal length / entrance pupil diameter
So if you can estimate both the entrance pupil diameter and the focal length, you can estimate the maximum aperture. Example: a 100mm lens with a 35mm entrance pupil is about f/2.8.
But if you do not know the focal length or field of view, you do not know enough to calculate how much light the lens gathers overall. Entrance pupil size alone is insufficient.
A practical rough method is:
- set the lens to maximum aperture
- focus a distant object onto paper
- measure the image distance to estimate focal length
- measure the apparent entrance pupil diameter
- divide focal length by that diameter to estimate the f-number
This will be approximate because the correct reference plane is not obvious, and the aperture shape/pupil magnification can introduce small errors. So yes, measuring the entrance pupil helps, but you also need focal length, and the result is only an estimate.
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