Thanks for the explanation, Frantisek. It's a keeper. The part about possible corrections is quite interesting. BTW I noticed that the panorama software I use (PhotoVista) has facilities to allegedly correct coloured fringes (CA-like) in the photographs. I don't know how it works exactly. All the best
Jens Bladt mailto:[EMAIL PROTECTED] http://hjem.get2net.dk/bladt -----Oprindelig meddelelse----- Fra: Frantisek Vlcek [mailto:[EMAIL PROTECTED] Sendt: 23. maj 2004 00:07 Til: Jens Bladt Emne: Re: Today's Question #1: CA Saturday, May 22, 2004, 9:56:23 PM, Jens wrote: JB> Joe JB> CA is (AFAIK) caused by the fact, that glas does diffract different colors JB> (wave lengths) differently (like a prisma). This means in practice, that JB> it's not possible for all colours in a photograph to be equally sharp in the JB> same surface film-plane. This is why they invented the (expensive) JB> APO-lenses, that are not sfaerical (parts of a ball), but asfaerical and JB> especilally made for avoiding/minimizing Chromatic Aberations. All this JB> means, that it's not just the fringes that causes problems - it's all over JB> the picture. But it's indeed very notisable near contours/edges. JB> All the best. JB> Jens Hi Jens, you have mixed up two different subjects ;-) CA is two kinds - lateral and longitudinal. ASP elements (aspheric) aren't there to reduce CA, they just allow the designers to design a lens with either fewer elements (e.g. SMC FA 2/35mm ASPH) to perform like a lens with more elements, or design a lens to a higher degree of correction (of some aberration, like spherical, etc.). Usually, it's said one aspheric element performs like two traditional elements in a design. APO glass is a long known concept. Eeach glass type has different refractive & dispersion characteristics. Even in old designs like Tessar they used several kinds of glass with quite different index. Nowadays, the "APO" (TM) is used in marketing, not denoting the kind of true apochromaticism one would except. Even so, apochromatic lens can be (and mostly is) truly apochromatic only in some conditions - e.g. over a given focusing distance, given zoom position. Apochromatic lenses like Apo-Tessar were there even in the 1940s... Apotessar doesn't have AFAIK any of the modern glasses, but it's a careful design with a very modest aperture (f/9). Truly catadioptric (mirror) lenses are free of CA as well. The new consumer APO lenses aren't truly apochromatic, probably the correction is only for a very narrow wavelength spread, even if for three colours. So there is still some residual CA. Truly apochromatic and superachromatic (correction for four wavelengths!) are corrected over a much wider spread of wavelentgh. Example - the Pentax Ultra lenses (4.5/85mm and 5.6/300mm, if I remember correctly), used for IR and UV photography. But as I said, CA is two kinds: Longitudinal - different colours of light focus on different planes. E.g. only the green light will focus correctly, other wavelengths will be focused slightly before or behind the focal plane. This is the most pronounced for telephotos. This is impossible to correct in postprocessing, as you cannot make e.g a fuzzy green channel sharp. Lateral - all wavelengths coincide on the same focal plane, but each has different image size! So the green image is sharp, as is the blue image, but they are different size! This can be corrected to some degree in software postprocessing by shrinking or enlarging only the respective colour channels. Hope this helps. Look on the web for more answers. You will also see that most 3rd party lens makers information on the so-called APO is rather scarce, not even giving the wavelengths the lens is corrected for, and for which spread in the wavelength it is corrected. Well, I can understand that, they can sell a lot of lenses named APO even if they aren't truly apochromatic or just for a small degree of zoom range/focusing distance/colour spread. Best regards, Frantisek Vlcek
