# Talk:Entrance pupil

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## Reqdiagram

I'd like to see a diagram showing a cross section of a lens with dotted lines going to the image of the aperture. —Ben FrantzDale (talk) 20:35, 27 July 2008 (UTC)

## Perspective Geometry

" When the optical system is physically rotated about its entrance pupil, the perspective geometry of its image does not change." -- this is false. the perspective geometry depends on the objects in the field of view, not on the figure of the optics. see for example the image at http://www.handprint.com/HP/WCL/IMG/LPR/pirenne.jpg, where the perspective geometry changes from central perspective to two point perspective simply by rotating a pinhole camera around the aperture. a correct formulation might be, "the projective geometry does not change ... " etc., but that is only the trivial claim that the optical path is not distorted by rotation. i have deleted the statement. Macevoy (talk) 15:29, 20 November 2010 (UTC)

I'm not sure whether your main point is correct, but if those two images are complete (not portions of larger images) they clearly do not differ merely by rotating the camera around the center of the aperture. If they did, the building could not be centered in both images.--Srleffler (talk) 05:43, 21 November 2010 (UTC)
Indeed. I too don't follow the initial question, but the example looks like two images with the same entrance pupil but different image planes. That is, they look like the same image up to distortion by a homography. —Ben FrantzDale (talk) 15:21, 22 November 2010 (UTC)
Tilt_(view_camera)#Back_tilt.2Fswing
Look at the vignetting - the camera back has been swung about a vertical axis.
--195.137.93.171 (talk) 09:58, 15 July 2013 (UTC)
i.e. the camera is not a pinhole camera as described above, and the rotation is not a rotation of the entire optical system about its entrance pupil.--Srleffler (talk) 01:28, 16 July 2013 (UTC)

## Location and size calculations?

I've been looking high and low for authoritative formulae for entrance pupil position and diameter but have come up dry with equations that match ZEMAX's. Its documentation claim it is using the paraxial position and diameter, yet doesn't give an equation and my attempts to compute those values paraxially fail to agree with ZEMAX in all but the trivial cases. What's a good reference for this stuff? —Ben FrantzDale (talk) 03:22, 10 April 2014 (UTC)

## Location of the pupil

"The entrance pupil [...] , it lies behind the first optical surface of the system." The pupil can be located well beyond the first optical surface, which I agree includes the fact it is behind the first optical surface, but formulated this way the sentence can mislead some readers to interpret it as that "behind the first optical surface" is its sole location. I suggest to remove "it lies behind the first optical surface of the system." or reformulate it: "[...] it lies beyond the first optical surface [...]".Kogatana (talk) 00:22, 1 February 2016 (UTC)

The full sentence reads, "The entrance pupil is usually a virtual image: it lies behind the first optical surface of the system." I don't see how to make that clearer. The main point here is that the image is usually virtual. It doesn't matter whether the pupil is just behind the first surface, or far behind it. Behind first surface = virtual.--Srleffler (talk) 06:13, 1 February 2016 (UTC)