Re: diffraction curiosity questions
- From: Helpful person <rrllff@xxxxxxxxx>
- Date: Thu, 13 Mar 2008 05:08:28 -0700 (PDT)
On Mar 13, 3:34 am, murrayatuptowngall...@xxxxxxxxx wrote:
OK, sorry.
It was a long-winded question. I was kind of looking for an answer
like:
A) I oversimplified a complex subject.
B) Yeah, I nailed it.
C) I misunderstood and made too many assumptions.
It's not for a class, by the way. I couldn't sleep the other day and
that's what I was thinking about. I figured the best kind of Dr. to
talk to about that problem would be here. Born & Wolfe was one of the
books I was reading a year or so ago. I got the big picture but not al
lthe details.
Really all I wanted to know (maybe I can make it fit category A B or
C), were the following, which might better be asked in a photo forum
but for all the old wive's tales, urban legends & krap that gets
passed around there. :O)
1) Am I correct in insisting that pinhole photography works for all
practical purposes solely BECAUSE of diffraction?
2) If Q1 is true, is diffraction fundamental to image formation with a
lens or is it reduced to merely an undesirable side effect to be
controlled like aberrations (I believe diffraction is not an
aberration).
I hope that's more acceptable. If not, I'll keep reading here & there.
Murray
1. Pinhole photography works despite diffraction. If you consider
rays coming from an object point their spread is limited by the
pinhole size. Hence initial thought would lead one to believe that
the smaller the hole, the sharper the image. (Ignore brightness for
now.) However, considering light from a single object point,
diffraction at the pinhole spreads out the light making the image
larger. To optimize the geometry you need to arrange for the
geometric (ray) image size to be similar to the diffracted image
size. (I believe that for best results the pinhole size is equal to
the diffraction size. Others please confirm.)
2. I'm not sure what you mean by "fundamental to image formation".
It could be argued either way. However, diffraction if an important
factor to take into account when designing (and using) well corrected
lenses. It is because of diffractive effects that the lens acts as a
low pass (spatially) filter. Hence it always needs to be considered.
Diffraction will reduce the spatial response of an imaging system, in
some cases limit it (as a filter it results in a high frequency cut
off; see MTF) and in others result in unwanted stray light.
Hope this is helpful!
.
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