> > From: "Anthony Farr" <[EMAIL PROTECTED]> > Date: 2006/06/19 Mon PM 01:42:34 GMT > To: "'Pentax-Discuss Mail List'" <[email protected]> > Subject: RE: Asahi ghostless filter > > > > 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.
That would, I think, have to be the case. It was the basis of my comment, as one curve would not apply to all lenses. As far as I can see..... m ----------------------------------------- Email sent from www.ntlworld.com Virus-checked using McAfee(R) Software Visit www.ntlworld.com/security for more information -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
