"rings" on photos. What could be the cause?

Hi. I've been playing around with pinhole photography, and I'm still in the
mode of figuring it all out.

Recently, I decided to change the focal length from 110mm to about 35mm. I
have an old Agfa Bily, and just removed the entire front end except for the
'accordian' part, the pinhole lines up with the front of the camera.

In doing so, I made another pinhole. But now I have tried 2 pinholes with
the new modified focal length, and I'm getting 'rings' on the photo
(http://soupisgoodfood.net/temp/pinhole.jpg, sorry for the poor photo, I had
to take a digi of it using my monitor as a lightbox :).

I'm guessing I didn't see the rings before I changed the focal length
because it fell outside the frame.

What do you think is causing it? Pinhole too small/refaction of light? The
fact that I made it from heavy aluminium foil (one of those pie trays that
comes with frozen pies to be exact). The "drill" method?

Thanks for any input.
Justin.
Received on Thu Jul 4 01:13:08 2002