# Diffraction determined by entrance pupil or exit pupil?

Started 1 month ago | Questions thread
Diffraction, Fourier and Focal Planes
1

AiryDiscus wrote:

Jack Hogan wrote:

AiryDiscus wrote:

This posting pattern is tiresome, destructive and wasteful.  A thoughtful contribution is made:

Jack Hogan wrote:

When dealing with diffraction in Fourier Optics one takes the complex-lens as black-box approach, where the Entrance and Exit pupils represent the terminal properties of a complex lens simplified down to just those two elements.

Someone on an ego trip doesn't take the time to understand what is being said or the context, yet questions the premise with a peremptory, often off topic, edict:

No, this is not what the pupil is in physical optics. Physical optics inherits all elements, assumptions, and definitions from geometric optics. Superset, not alternative set.

The response is easily rebutted and the conversation brought back to the original topic

That may very well be, but In Fourier optics the source I mentioned is pretty clear, referring to the terms in italics above (Goodman ch. 6):

... the "terminals" of this black box consist of the planes containing the entrance and the exit pupils.

Caught, to save face the message and the messenger need to be thrown into question:

This text does not appear in chapter six of the current edition of the book. I will assume you're using an illegally acquired copy of the second edition, which is easy to find online.

To what purpose did I just waste time to show that I knew what I knew and that the sources I said supported what I said, supported it? For the record, my source is Introduction to Fourier Optics third edition, which I have owned and read for several years. It says exactly what I said it says. Same with Goodman’s phase shifting plate analogy, which may be ‘buzzwords’ to some but to me have always been useful in visualizing the math. From my own personal library of real books:

Diffraction and Fourier Optics, Introduction to Fourier Optics 3rd edition, Goodman ch 6

Enough wasteful diversions, back to substance. Where the rubber meets the road with regards to this sub-thread is the skill-testing question that was answered incorrectly below:

In the meantime I have a skill-testing question for you or anyone interested:

Let's say you have such an exit pupil and related function, a lens of focal length f and f-number N, focused at infinity. You use Fourier Optics to propagate the field at the exit pupil to the focal plane. How far precisely is that from the exit pupil? From the principal plane?

In the case that you wish to invoke the Fourier transforming property of a lens and use what I would call a focusing propagation, simply taking an FT, then the distance is one focal length.

Goodman would beg to differ. Vectors are once again being placed wantonly and without thought, just like some of the others I lamented about earlier , QED. Where is focal length measured from?

There is, again, no work to show (this is the fourier transforming property of a lens), unless you would like me to recreate proofs that have been known for what is now a very long time.

I encourage you to read the whole book, and not skim it around formulas you want to lift without context. Or take a course in the topic.

Prompt: using the variables at hand and any others that may be needed,

zi = f + ...

Of course the context is photography.

Jack