On 21 Nov 2001, at 22:34, Raimo Korhonen wrote: > Well - some are more apochromatic than others but basically Mike is right - and > all manufacturers claim that their APO lenses are apochromatic (but how much?) - > including the famous Kern Macro Switars for Alpa. Everything is relative and IMO > no lens is absolutely apochromatic. And Sigma�s APOs are more apochromatic at > tele end than the DL versions.
Hi, I am not sure where the problem lies in that the term APO simply indicates that a lens will be in focus at three discrete and distanced frequencies within the visible spectrum. Where the problem lies is that manufacturers give no indication of the degree of focus error either side of these points. If you understand this then the qualification APO should be acceptable. See the extract following from rec-photo/lenses/faq by By David Jacobson [EMAIL PROTECTED]: "Q31. What do APO and Apochromatic mean? A. The distance behind the lens at which monochromatic light (light of a single wavelength) comes to focus varies as a smooth function of the wavelength. If this function has a zero derivative in the visible range, and hence if there are two wavelengths at which the light comes to focus in the same plane, the lens is called achromatic. If there is a higher order correction, usually with the result that 3 or more visible wavelengths come to focus at the same distance, the lens is called apochromatic. Some authorities add more conditions. Apochromatic lenses often contain special low-dispersion glasses. APO is an abbreviation for apochromatic. It is frequently asserted in the rec.photo.* newsgroups that marketeers use the terms apochromatic and APO rather loosely." Cheers, Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications.html - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
