Just a note to point out that the 12mm/5.6 Voiglander lens is definitely a retrofocus lens. The only lens near this focal length I know of that is symmetrical is the Hologon, and that is a 15 (or later, 16mm). The 15mm/4.5 Voigtlander is also retrofocus.
If the two Voigtlander lenses had not be designed as retrofocus lenses, metering on the M6 and later cameras would have been impossible, and the two would have been totally useless on the M8 and M9. Just try using the Hologon on an M9. Or, for that matter, the 21mm/3.4 or f/4 Super Angulon lenses, which are also essentially symmetrical. That's also the problem with the 21/4.5 Biogon. The 21/2.8 Biogon is of definite retrofocus design.
An easy way to tell if a lens is retrofocus (or telephoto, for that matter), is to look through the front of the lens at a given aperture, and then through the back. If the aperture looking through the back is noticeably larger such as in the case of the 12mm, it is retrofocus. If it is noticeably smaller, it is telephoto. If similar, it is essentially of 'normal' design, and in the case of classic wide-angle lenses, usually symmetrical.
This take nothing away from the main points of the article, which clearly show which lenses can be used easily and with no extra contortions as opposed to those which, like the 12mm, may be retrofocus but are of insufficient retrofocal ratio to avoid the problems of imaging on digital sensors which are intolerant of sharply angled imaging rays (nearly all of them).
Henning
