On 04-Sep-2002 [EMAIL PROTECTED] wrote: > Thanks for the advice on the center filter for the Metrogon. I actually > had a different problem in mind. As I understand it a piece of flat glass > adds spherical aberration. Someone in this group reported that the > Metrogon was calculated with this in mind by including compensating > aberration of the opposite sign so optimum resolution would be reached when > one of the center filters was in place.
I assume that the center filter is used in front of the Metrogon? If so, then, for the photography of distant objects, the mere fact of having a flat uniform thickness filter in front of the lens does NOT introduce spherical abberation. For distant objects, the arriving light rays are very close to parallel -- the effect of a flat uniform thickness piece of glass on parallel light rays is to slightly shift the rays in a parallel manner -- with an object at infinity this isn't noticeable. Conversely, if a flat uniform thickness piece of glass is inserted into a beam of convergent rays, spherical abberation will be created. This would be the case for a filter behind a lens, or a filter in front of a lens when the lens is focused on a close object. If you are doing closeups or using a filter behind a lens, you should focus with the filter in place -- it changes the apparent distance. Compared to other filters, a center filter has some complicated effect on abberations. If you a lens without a center filter, the light rays that reach the film have arrived with equal probability over the front of the lens (at least the part of the glass used at the taking aperture). If you use a center filter, then the light rays at the edge have a higher probability of reaching the film. If the abberations contributed by the outer radii are different from the inner, then the mean abberation will be changed. I don't think there is any simple way to predict this effect. For a really wide-angle lens, the improvement in exposure uniformity due to using a center filter is likely to be more important. --Michael _______________________________________________ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
