Quote Originally Posted by Q.G. View Post
Seen from under an angle, not from straight behind, a circle appears as an ellips, is smaller.
Make the hole ellips shape, and from the side the long axis points to, the effect is negated. But only from that side, and only at a certain angle/distance from the image's centre.

I don't know how that would creat astigmatisme, except that the shape is echoed in the pencil of light's shape. But that's not astigmatism.
Hmm - interesting. I'm still confused on the answer to my original question though. I know that the hole isn't supposed to be too small because you'll run into problems. I just don't understand why a pinhole optimized for a focal length of 100mm couldn't be used with a 200mm camera - since the smaller the hole, the better (up to a certain point).