At 16:31 13/10/97 -0600, someone wrote:
>The role of diffraction in your message is unclear.  I'd really like to
>hear how people think a pinhole image is actually formed.

I'm not a physics specialist, but I learned some a long while ago, so here
goes...

Let's start in a classical world where light travels in straight lines from
a to b. Under these circumstances each point on the film plane is exposed to
photons (light particles) from the points on a cone extending out through
the pin hole to the real world. (I tried sketching this in ASCII but it just
looks silly)

Hence, the smaller the pin hole the sharper the image. This is because the
angle of the cone is smaller, and each portion of the film is exposed to
light from a smaller area.

However light is not made up of simple particles. Photons have wave like
properties too, and it it this which results in diffraction. When a photon
passes through a very small hole it doesn't always travel in a straight
line. It can be diffracted by interacting with the edges. This reduces
sharpness since a point on the film can now receive light from ajacent
points which has been diffracted.

So when we make a pinhole we have to balance these two factors. Large
pinholes have very little diffraction, but poor resolution for geometric
reasons. Very small pinholes are dominated by diffraction from the edges.
This leads to the rather strange formulae we have been seeing on the list.

While I was thinking about this I noticed another factor. As you move off
the axis of the pinhole it stops being round and becomes progressively more
elliptical. This should result in reduced sharpness at extreme (wide)
angles. Has anyone noticed this?

Keith.
------------------------------------------------------------
Keith Dowsett         "Variables won't; constants aren't."

E-mail: kdowsett@rpms.ac.uk  or kdowsett@geocities.com

WWW: http://kd.rpms.ac.uk/index.htm
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