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exit pupil/nodal point question
- Thread starter Wanderer
- Start date
Chuck Westfall(Canon USA)
pro member
Sorry, we've asked before but Canon Inc. has decided not to release this data.
Best Regards,
Chuck Westfall
Director/Media & Customer Relationship
Camera Marketing Group/Canon U.S.A., Inc.
Best Regards,
Chuck Westfall
Director/Media & Customer Relationship
Camera Marketing Group/Canon U.S.A., Inc.
D
Doug Kerr
Guest
Hi, Wanderer,
Note that, with the lens focused at infinity, the second (rear) nodal point will be precisely the focal length in front of the focal plane. Thus, for EF lenses, the distance from the 2nd nodal point to the flange reference surface will be:
f-44.00 mm
where f is the actual focal length of the lens. (For focus at infinity, that will generally be the "marked" or "advertised" focal length.)
(44.00 mm is the "back flange", or "register", distance for the EOS camera series, the design distance from the mount flange reference surface to the focal plane.)
Yes, for a 35-mm lens (for example), that will be in thin air behind the physical lens. This is what "retrocus design" makes possible. (Otherwise, we couldn't have short focal length lenses on an SLR.)
One can get a pretty good approximation of the location of the exit pupil visually. If you look into the aft end of the lens, at what you think is the "iris", what you are really seeing is the the virtual image of the iris as seen from behind the lens, which is in fact defined as the exit pupil. If you can visually estimate how "deep" that is into the lens, you have your answer.
Note that to be certain you are looking at the (virtual image of) the iris and not some other circular outline, it is helpful to have the lens stopped down.
You can do that by having the lens on the body, setting a farily small aperture, pressing the DoF preview button to make the lens stop down to that aperture, then (while still holding the DoF preview button), dismounting the lens. The iris will stay where at the chosen aperture.
Yes, this is a little tricky to do if the 2nd nodal point is to the rear of the entire physical lens.
Note that, with the lens focused at infinity, the second (rear) nodal point will be precisely the focal length in front of the focal plane. Thus, for EF lenses, the distance from the 2nd nodal point to the flange reference surface will be:
f-44.00 mm
where f is the actual focal length of the lens. (For focus at infinity, that will generally be the "marked" or "advertised" focal length.)
(44.00 mm is the "back flange", or "register", distance for the EOS camera series, the design distance from the mount flange reference surface to the focal plane.)
Yes, for a 35-mm lens (for example), that will be in thin air behind the physical lens. This is what "retrocus design" makes possible. (Otherwise, we couldn't have short focal length lenses on an SLR.)
One can get a pretty good approximation of the location of the exit pupil visually. If you look into the aft end of the lens, at what you think is the "iris", what you are really seeing is the the virtual image of the iris as seen from behind the lens, which is in fact defined as the exit pupil. If you can visually estimate how "deep" that is into the lens, you have your answer.
Note that to be certain you are looking at the (virtual image of) the iris and not some other circular outline, it is helpful to have the lens stopped down.
You can do that by having the lens on the body, setting a farily small aperture, pressing the DoF preview button to make the lens stop down to that aperture, then (while still holding the DoF preview button), dismounting the lens. The iris will stay where at the chosen aperture.
Yes, this is a little tricky to do if the 2nd nodal point is to the rear of the entire physical lens.
Thanks, Doug,
Before posting I had finished the exercise of calibrating my lenses at 50 mm and above.
I did this by setting near and distant vertical calibration points and moved the camera to and fro on a focussing rail until central, left and right positions of those calibration points coincided, or close enough.
If I had known the distances, I would have saved a lot of time, and so I thought the suggestion might help others.
Wanderer
Before posting I had finished the exercise of calibrating my lenses at 50 mm and above.
I did this by setting near and distant vertical calibration points and moved the camera to and fro on a focussing rail until central, left and right positions of those calibration points coincided, or close enough.
If I had known the distances, I would have saved a lot of time, and so I thought the suggestion might help others.
Wanderer
D
Doug Kerr
Guest
Hi, Wanderer,
I'm not exactly able to follow your procedure. I'd be interested to hear more about it.
Which point do you believe that gave you, the location of the exit pupil or the locaton of the one of the nodal points? (I mentioned the 2nd nodal point in my reply as I thought perhaps that was what you were interested in since you also mentioned the exit pupil.)
It sounds a lot like a test that would give the location of the entrance pupil (again I can't tell for certain from your description).
Are you interested in this as the location of the best pivot point for multi-shot panoramic photography? Then it is the location of the entrance pupil you would want.
Best regards,
Doug
I'm not exactly able to follow your procedure. I'd be interested to hear more about it.
Which point do you believe that gave you, the location of the exit pupil or the locaton of the one of the nodal points? (I mentioned the 2nd nodal point in my reply as I thought perhaps that was what you were interested in since you also mentioned the exit pupil.)
It sounds a lot like a test that would give the location of the entrance pupil (again I can't tell for certain from your description).
Are you interested in this as the location of the best pivot point for multi-shot panoramic photography? Then it is the location of the entrance pupil you would want.
Best regards,
Doug
D
Doug Kerr
Guest
Hi, Wanderer,
Yes, the proper pivot point for parallax-free panoramic phtography (PPPPPP) is located at the entrance pupil of the lens.
We can find that by visual examination, looking into the front of the lens (just as I described earlier for the exit pupil. where we would peer into the rear of the lens).
Best regards,
Doug
Yes I want the location for multi-shot panoramic photography that compensates for parallax.
Yes, the proper pivot point for parallax-free panoramic phtography (PPPPPP) is located at the entrance pupil of the lens.
We can find that by visual examination, looking into the front of the lens (just as I described earlier for the exit pupil. where we would peer into the rear of the lens).
Best regards,
Doug
Kirk Darling
New member
Doug Kerr said:Hi, Wanderer,
Yes, the proper pivot point for parallax-free panoramic phtography (PPPPPP) is located at the entrance pupil of the lens.
Ah, I was just wondering what the acronym was for that term. Now I can use it in discussion. Only problem is that people will think I'm Mel Tillis.