Mike Wilson wrote: > > A curved filter would be useless for general use. >
While that may seem a logical claim, curved filters were (are?) in fact made by our very own Asahi Optical Co. for the very reason that they eliminated ghosting. Although curved they have no optical properties, the surfaces are parallel just as they would be if the filters were flat. A ghostless filter is a double meniscus with a zero dioptre. What makes these filters ghostless is that the ~image-forming~ rays always go through the glass at 90 degrees, unlike a flat filter where the angle of incidence increases more and more as the rays are collected further outwards from the optical axis. The divergence from the perpendicular is what causes ghosting, and is why ghostless filters work. They may appear to be full of distortion on a casual inspection, just like an old style watch glass, but the light rays that look distorted ~are not~ image forming rays. I have always wondered if these filters were matched to particular lenses or lens types, e.g. very curved for very wide angle lenses and almost flat for long focal lengths. Regards, Anthony Farr -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net
