# Diffraction determined by entrance pupil or exit pupil?

Started 1 month ago | Questions thread
Re: Exit Pupil Geometry

Joofa wrote:

Jack Hogan wrote:

bclaff wrote:

AiryDiscus wrote: ... I find pictures are much more illustrative than equations. Here's a lens at infinity, with an illustrative choice of stop location, one half focal length from the lens:

This places the EP at the stop, -0.5f from the lens, and the exit pupil at -f, sized to be .66 the stop diameter,. D_stop. The pupil magnification is .66. ...

I have trouble with that diagram.

With focal lengths meeting at lens that's a lens where the principle planes coincide, eg. inter-nodal distance is zero; fine.

But if pupil magnification is going to be .66 (I'll use 2/3) then the pupils will not be as you placed them.

Yeah, the position of the Exit Pupil doesn't quite look right. Pupil magnification of 2/3 should mean that the Exit Pupil is 1.333f from the focal plane, no?

Right, Bill C. has rightly pointed out the issue.

To answer your question, in the current setting the distance between exit pupil and image plane will be 2/3 f.

Right, right, I got signs mixed up.  Just mp*f.

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