Archived posting to the Leica Users Group, 1999/02/01
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>snip >> Please note that all Leica M lenses below 35mm are now retrofocus lenses, >> although only to a slight degree in comparison to SLR lenses. This is to >> allow the M5 and M6 metering. On the positive side, this makes it easier to >> design a lens that has even illumination over the field, but on the >> negative, it makes it harder to control distortion. As a consequence, the >> new 21/2.8 ASPH has more distortion than the 21/3.4. These differences are >> slight, but they do exist, and might possibly make some difference in your >> shooting. >> >> * Henning J. Wulff >snip > >Thanks for the valued info. The retrofocus issue and been out of in my >mind for years and I trust you have helped clarify it. >It's my impression that retrofocus somehow means: telephoto turned >backwards. If this might be the case how can something be slightly retrofocus? >As to the distortions issue I am recalling Erwin's articles which I'm >pretty sure are convincingly to the contrary, but I'll go check now. >thanks again, >Mark Rabiner The terms 'retrofocus' and 'telephoto' are bandied about rather loosely, but generally are used to describe optical systems which respectively have their elements farther away from the film plane and closer to the film plane than what might be called 'standard' construction lenses. In telephoto lenses this means also that the entrance pupil is larger than the exit pupil of the lens. If you hold the lens far away from you (theoretically, at infinity), the clear aperture of a telephoto lens seen from the front is larger than the clear aperture seen from the back. On a retrofocus lens, it's the opposite; the clear rear aperture is larger than that of the front aperture. A 'slight' retrofocus lens is one in which the rear aperture is 'not a lot' larger than the front aperture, and an 'extreme' retrofocus lens is one in which the rear aperture is 'a lot' larger than the front aperture. If you look at a 15mm SLR lens, or even more so a 6mm/2.8 fisheye, the aperture seen from the front is tiny (6mm divided by 2.8, or just over 2mm), while the rear aperture looks as big at f/2.8 as that of a 50mm at 2.8. Linear distortion is due to a variation in magnification over the field which renders straight lines in the object as curved lines in the image. A symmetrical system (which pretty much describes the SA) has intrinsically little or no distortion. A completely symmetrical system has none. A retrofocus lens, for a number of reasons, generally has a distortion patter which shows slight barrel distortion at around 8 to 10mm off axis on a 35mm frame, and then to compensate shows some pincushion distortion at around 18mm off axis. This leads, in many cases, to a distortion which might be called 'moustache' shaped. Most retrofocus lenses show this to some degree, and it is a compromise reached to make sure that neither barrel or pincushion distortion predominate and make the image unusable. Some SLR lenses, especially retrofocus lenses of the 60's, and a lot of zoom lenses, have very serious distortion problems. The 21/2.8 non-ASPH has some of this 'moustache' distortion, but a lot less than almost all SLR lenses; the 21/2.8 ASPH has even less, and the 21/3.4 SA has even less, to such a degree that it is virtually distortionless, whatever that means. In any case, it has a similar level of distortion as that of my Hasselblad SWC Biogon, or that of my various 4x5 wideangle lenses, all of which are almost symmetrical designs. The only reason these lenses are not symmetrical designs is due to the fact that they are not for imaging at 1:1, but for imaging distant objects. Apo process lenses (a dying breed) optimized for 1:1 are all symmetric designs, and have no distortion. Architectural photography is my job, and I depend on lenses with this sort of coverage. Believe me, a 21/3.4 SA has less distortion than a 21/2.8 ASPH. At the moment I still have all 3 21's, and I have determined their capacities for myself over the last year. Depending on your priorities, either the 21/3.4 or the 21 ASPH is best, but all three are extremely good, and much better than nearly all SLR lenses. * Henning J. Wulff /|\ Wulff Photography & Design /###\ mailto:henningw@archiphoto.com |[ ]| http://www.archiphoto.com