From: Joe Smigiel (jsmigiel@kvcc.edu)
Date: 12/19/02-03:42:46 PM Z
One of the points I (and several others) tried to make is that the discussion was becoming muddled because people are confusing the terms wide-angle and telephoto with the terms short and long focal length, respectively. The former refer to lens designs while the latter have to do with focal distances.
I have a 210mm Schneider Angulon lens which is a wide-angle design and covers an 11x14 negative. It is the normal focal length for a 5x7 camera (based on the film diagonal) and a short focal length for the 11x14 (based on its wide-angle design). I have another 210mm lens which doesn't even cover an 8x10. It is still a normal focal length for the 5x7, but since it is a normal or standard lens design, it vignettes on anything larger than a 5x7. It is not a wide angle lens because of its design. If I put the lens on a 4x5, it is still a normal lens design and is not a telephoto. However, on that 4x5 camera it would be a long focal length. An image at infinity will still come into focus at 210mm with that lens regardless of the film format. A telephoto lens would bring the image into focus at a distance less than its focal length. The terms are not interchangable.
Colloquially, it may not make much difference, but practically it does in the field or when someone is buying equipment. A quality wide-angle lens is going to cost more and present different operational problems than a normal lens of the same focal length. I'm not trying to argue here, just trying to be helpful and clarify terms because the discussion was mixing up optical effects such as sharpness with angle-of-view, perspective, focal length, optimum pinhole diameters, coverage, etc.
As Katherine states, the term "normal" focal length is based on the film diagonal, as are the terms "short-" and long-" focal lengths. The terms wide-angle and telephoto are not interchangable with them and do not mean the same thing.
We can make short focal length pinhole cameras, normal ones, and long focal length cameras. They typically would have wide, normal, and narrow angles of view respectively (unless we vignetted the image). But we cannot make wide-angle or telephoto pinholes.
If the pinhole shim is thick enough to vignette the light entering the hole, then the angle of coverage is limited by that thickness and the light will be prevented from covering the film diagonal. Darryl wanted to know how to make a circular image on a piece of film within a pinhole camera. Vignetting with a thick "shim" and a long focal length is one way to do it. The other way is to make the focal length very, very short so that the diagonal is not covered due to focal distance. They have different consequences in terms of image sharpness, size, and angle of view. Masking to produce a vignette is a third way. For example, most 35mm cameras do this by limiting the frame at the film plane so that the image appears on the film at a size of 24mmx36mm. Half-frame cameras vignette the image further. The coverage is still circular as a cone of light enters the camera.
There are a lot of other things affecting pinhole sharpness and Shannon has cited one aspect: there is an optimum diameter for specific focal lengths. This is due (as I understand it) to diffraction of the light waves as they pass through narrow or thin openings. Therefore each focal length will have an ideal aperture. Anything larger produces unsharpness due to the size of the circles of confusion (apt term) and anything smaller causes diffraction effects. If Darryl wanted the sharpest possible image at a certain focal length AND wanted to limit the coverage to a circle on a specific format, using the optimum diameter and some sort of vignetting (thick shim or restrictive circular mount) solves the problem.
I don't know why a thick shim would cause reflection problems in Katherine's camera. If the image is unsharp with a thicker shim, it might be that the optimu
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