Shel wrote: > Now this I'd like to hear more about. Can you elaborate on this point, > specifically, how do manufacturers get away with labeling a lens as > Apochromatic when it isn't? Is the term "APO" used to imply that a lens > is apochromatic, when, in fact, it's just a model name given to the > lens? Which lenses that are designated as "APO" lenses are not, in > fact, apochromatic? Which are? Just a few examples would be > appreciated. > > Would a lens have to be designated as "apochromatic" to be apochromatic, > as opposed to being labeled as "APO", which may not be apochromatic?
Taking your last question first, APO = apochromatic. An apochromat is a lens that images three spectral regions at identical size. Early lenses could be corrected for blue light only, because early emulsions were blue-sensitive. For a number of years lenses were corrected for two colors letting the third fall where it must. This was especially a problem with telephotos, as errors were amplified. With the introduction of Fluorite, ED, and partial anomalous dispersion elements, teles could be corrected much better for three colors, bringing correction of the tertiary spectrum to a tiny fraction of what was once accepted. As soon as these glass types came into use, telephoto lenses got markedly better quickly. In practical terms, determining whether a lens is apochromatically corrected or not is a little like claiming that a person is six feet tall exactly. That might be true to a tolerance of a quarter inch; or even a millimeter; but can it be said to be true to a tolerance of 1/10,000th of a millimeter? Obviously not. Same way with the "same size" images in three colors. You have to determine the tolerance you'll accept. So what if you reduce the red error to 1/100th of what it was in an achromat, but it still has five times the error compared to the blue? In practical terms, lensmaking companies all set their own standards for what constitutes apo correction. Since many photographers have a vivid memory of the time when apochromatic correction from the use of extra-low dispersion glass radically improved long teles, "apo" has become a hallowed sales buzzword. In the early days it only meant that the designer attempted to correct for the tertiary spectrum, not that he actually succeeded. So some companies slap the word on virtually any lens they think is any good. Other companies set much more stringent standards for themselves. When Leica puts the term "apo" on a lens, you can bet it's very well corrected well into the red, 900nm or more probably. Zeiss is also very strict about the term. Pentax doesn�t even bother to use these buzzwords even when it can: several of its lenses are asphericals and several use ultra-low dispersion glass and could quite justifiably be labeled as apos, but aren't. So it's usually not a question of "is it apo or not," but of "how apo is it?" The only lenses that meet the true definition, usually, are process lenses for repro work, and maybe a select few enlarging lenses. I'm pretty sure my $3,000 Carl Zeiss S-Orthoplanar is a true Apochromat. The Apo-El-Nikkor enlarging lenses made in the 1970s were. Are any camera lenses? A few can legitimately claim the designation. Most are simply called apo to make them sell better. Man, it's late, and all this is right off the top of my head, stream of consciousness. I need to start writing "FAQ sheets" for my newsletter and really do a thorough job explaining things like this. Then I would have to dive in and do it again fresh each time someone asks. Hope this helps. Sorry if it's semi-incoherent. --Mike - 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 .
