How can I estimate a lens’s exit pupil distance from the sensor plane?
Asked 4/9/2018
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I need an experimental way to estimate the distance from the sensor plane to a lens’s exit pupil, with accuracy of a few millimeters over roughly 10–70 mm. This is for lab work with an angle-sensitive sensor, where I want to calculate chief ray angles at different sensor positions.
I considered a simple thin-lens approach with a collimated beam, using:
exit pupil distance = (spot distance from image center) / tan(incident angle)
Is that approximation valid for finding the exit pupil position? If not, what simple experimental method can give a good estimate?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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As you know, the entrance and exit pupil is the location of the aperture stop. However as seen from both the front and the rear of the lens, the location of this iris diaphragm is elusive, because you are looking at it though lenses of unknown power. Thus what you are after is the location of the virtual image of the iris.
Try this technique: Set the lens so that it is elevated off the bench, perhaps on a stack of books. Stop the iris down to its smallest diameter. Illuminate the iris with a flashlight beam. Mark the center of the rear and front element lens with wax pencil, make a dot. You might choose to use a scrap of marked cellophane tape, instead.
Procure a magnifying glass, perhaps a 10X loupe (you may need less or more power for this task). Peer through the loupe held to your eye. Focus on the blades of the iris diaphragm, as seen from the rear of the lens. Now back off and focus on the spot you made on the surface of the exit lens. You were forced to move the loupe towards or away from the iris blades to obtain focus. The distance the loupe traveled reveals the location of virtual image of the iris (rear exit pupil). Repeat viewing from the front to discover the location of the entrance pupal.
The above procedure might be more accurate if you substitute a close-focusing SLR. Focus on the blades of the iris, then without changing the camera’s focus, back the camera until the mark of the lens is in-focus. The distance the camera traveled reveals the spacing between the last lens element and the location of the virtual image location of the pupil.
Hope this helps!
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
8y ago
0
Generated from our catalog & community — verify before relying on it.
Your thin-lens formula can give only a rough estimate. In a real photographic lens, the exit pupil is the virtual image of the aperture stop seen through the rear lens groups, so its position usually does not match a simple thin-lens model.
A practical way to estimate it is to measure the apparent position of the stopped-down aperture as seen from the rear:
- Set the lens to infinity focus and stop it down fully.
- Place the lens securely with the mount facing you.
- View the rear of the lens with a camera on a focusing rail, or with a loupe/magnifier.
- Focus on the apparent image of the aperture blades (the rear-view image of the iris, not the physical blades themselves).
- Measure the distance from that focus position to the lens mount/reference point, then convert to sensor-plane distance using the mount’s flange focal distance.
Another classical bench method is to project a 1:1 image of a ruler and use the geometry to infer principal/nodal locations, but that generally needs more setup and care.
So: for a simple, few-millimeter estimate, directly locating the rear virtual image of the aperture stop is the most practical method.
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