View Full Version : Glass pinhole.

08-06-2006, 06:24 PM
Does anybody knows what happens whith refraction, and how affects the image, when a 2mm glass interferes the negative?. The thing is that I need a 4x5 negative support for my new camera. Glass seems to be the best, but, how about the refraction?.


Jim Jones
08-06-2006, 09:20 PM
Because of refraction the image will be displaced longitudinally a fraction of a millimeter. Also, reflections from the glass might produce a faint double image. However, most 4x5 film is self-supporting in conventional film holders and in most improvised holders. The glass may be unnecessary.

08-07-2006, 03:43 PM
Thanks Jim. Iīll expreiment with it anyway, the results may be interesting. What about filters?. Whatīs the best stage to use them, and whatīs the best kind of?.

Thanks again.

Jim Jones
08-07-2006, 06:40 PM
Filters can be used anywhere from the light source to the film. Using them near the lens or pinhole permits the use of the smallest filters. Also, most readily available filters are most conveniently used near the lens. Filters mounted close to the film might not need to be as high quality as those mounted near a fine lens. Filters mounted between the light source and the subject can be of crude quality.

08-12-2006, 11:34 PM
If you have a piece of glass between lens/pinhole, as a filmplane flatness enhancer, this was done on some ancient Ansco LF folders and even Hasselblad did this on a Polaroid accessory.

The glass plate acts like a flat lens, and extends focal length by an amount equal to the glass thickness divided by the glass index of refraction.

Ordinary glasses have an index of refraction very close to 1.5...so for example a 3 mm flat plate between lens and film extends f.l. by approx 3/1.5 = 2 mm.

This is already addressed if you use ground glass focussing because you see the 'extended' f.l. on the ground glass. Similarly for filters...it you are NOT using ground glass and for some reason place filters behind a lens, if they are very thin, stopping down to a very small aperture may cover that for you.

08-13-2006, 08:50 PM
Thanks Murray, thatīs what I call info!. It seems thatīs the only effect to care about. Iīm I right?.