I've got a question for those of you who are knowledgeable about lens-to-camera interface theory. One of the advantages of rangefinder camera design is said to be eliminating the mirror box of SLR cameras and the resulting compromises in optical performance, especially with wide-angle lenses. To quote from the Mamiya 7 brochure: "A significant advantage of rangefinder camera design for the optical engineer is the elimination of the mirror box and the resulting shorter flange focal distance which permits placing the rear lens element close to the film plane. Various other restraints on lens design are also removed, providing far more versatility in coming up with a lens composition that approaches ideal optical theory." I own a Mamiya 7, and it's certainly true that with lenses like the 43mm and 65mm, the rear element seems to be backed right up almost against the film plane (in fact these two lenses protrude so far into the camera body that the instruction manual warns you, when mounting either of them, to take care that the rear lens rims do not touch the rangefinder coupling roller). Martin Silvermann, a Mamiya rep responding to a question at the Mamiya website (and before you discount his words as those of a salesman, remember that he's selling both rangefinder and SLR systems), noted: "The real virtue of any rangefinder system over an SLR system is in the inherent design advantages of RF wide-angle lenses. Since there is no mirror box to contend with, RF wide-angle lenses can be "straight" or "true" wide-angle designs, where the focal length is more in line with the actual film-to-optical-"center" distance (actually in most cases, the rear nodal point, for your optical buffs). SLR wide angles are "retrofocus" design, which leads to all sorts of compromises in order to allow focusing through the mirror system. In the end, the RF design will yield superior results, all other things being equal."

My question is, if all this is true (and I'm not suggesting that it isn't), how do large-format photographers get such sharp images, when the rear elements of their lenses may be at some considerable distance from the film plane? Doesn't there seem to be an anomaly here? Why doesn't this distance seriously compromise optical performance?

-- Dave Kemp (Kempda@worldnet.att.net), August 27, 1998

Answers

Ummm. LF cameras are in the same boat as rangefinder cameras. You have to buy a LF camera which can cope with moving the front and rear standards close enough for the lens you bought. If it is a super-wide LF lens like 47mm not all cameras can cope with it, or if they do they often need a recessed lens board so that you can get the lens close enough to the film plane.

As for the original statement about non-retro focus lenses being better, I guess it's true although I believe the "true" wide angles suffer a lot more from light fall-off. This is true of LF lenses and it was also apparently true of the Nikon 28Ti compact camera.

Apparently, even the Leica rangefinder 28mm, 24mm and 20mm lenses are retrofocus! That's my understanding anyway. Otherwise the Leica 24mm and 20mm would be really tiny lenses. But they aren't any smaller than the Leica 50mm. Compare the Contax 16mm Hologon which is a true wideangle and doesn't protrude from the body at all!

-- Chris Bitmead (chrisb@ans.com.au), August 27, 1998.

The last first. The distance from the lens to the film plane is going to vary with the focal length. The very small distance for a conventional WA lens on a 35mm camera has to do with its very short FL; it's just going to be close. If you take the ratio of focal length vs. distance from the rear element to the film, it will be the same for a given lens regardless of the FL, but the absolute distance is going to be more with LF lenses. Actually, there are types of aerial camera wide-angle lenses where the rear element is a field flattener element and is designed to fit right against the film, holding it flat also. There are both advantages and disadvantages to the "retro-focus" designs. Some aberrations are harder to correct but some others get better. It is easier to design a retro lens with a tilting exit pupil to increase the marginal illumination over the cos^4 law value. There are very fine WA lenses of both types. I rather suspect the company rep is touting the official company line. In general WA lenses are tough to design, especially if they are to have any speed. The very large front and rear elements seen on many is to reduce mechanical vignetting as much as possible. I hope this is helpful and not even more confusing.

-- Richard Knoppow (dickburk@ix.netcom.com), August 28, 1998.

Dave- You were headed straight toward answering you own question, but then at the end you veered off by thinking that it was the lens to film distance that was the important point. It is not. Some of the sharpest lenses are so called process lenses, used in the graphic arts industry, and these are often placed several feet from the film. The point your salesman was trying to make was the difference in lens design necessitated by the presence of a mirror box on reflex cameras. The focal length of a lens is measured from something called the rear nodal plane, to the film. On a single element thin lens this rear nodal plane is right in the center of the lens. On more complex lenses it could be anywhere, but on so called standard lenses it is usually somewhere inside the lens and often toward the rear. On a telephoto lens it toward the front of, or even in front of, the lens. On a reverse telephoto, or retrofocus, lens it is often behind the lens. If the point to which the focal length is measured is behind the lens, then the lens can (must) be further from the film. This allows space for, lets say, a mirror box. There are those who claim that a standard lens design offers better (sharper) lenses than either telephoto or retrofocus. Notice Rodenstock advertising these days that it is not necessary to buy telephoto lenses anymore as their standard lenses will do the job. Also Hasselblad is still selling their Superwide cameras (a camera that was designed specifically to use the 38mm Biogon lens (not retrofocus), and with which you must give up having a reflex camera) even though you can buy a 40mm retrofocus lens from Hasselblad that will fit their standard body. The Biogon lens couldnt be mounted to the standard body because the rear nodal plane is within the lens and this would have put to lens to close to the film to allow for the mirror, so they designed a special body with no mirror. But you know all this. The point is that its really not too important how far the lens is from the film. As to why LF photographers get such sharp pictures, that probably has to do with the size on the film.

-- Steve Pfaff (spfaff@hrl.com), August 28, 1998.