Lens-to-camera interface: rangefinder vs. SLR vs. large format

[Dave Kemp]

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 o ptical performance, especially with wide-angle lenses. To quote from the Mamiya 7 brochure: "A significant advantage of rangefinder camera design for the opti cal engineer is the elimination of the mirror box and the resulting shorter flan ge 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 opti cal theory." I own a Mamiya 7, and it's certainly true that with lenses like th e 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 tha t the rear lens rims do not touch the rangefinder coupling roller). Martin Sil vermann, 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 "str aight" 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 m irror 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 t heir 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 compromis e optical performance?

[Chris Bitmead]

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 f or 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 yo u 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 ligh t fall-off. This is true of LF lenses and it was also apparently true of the Nik on 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 rea lly tiny lenses. But they aren't any smaller than the Leica 50mm. Compare the Co ntax 16mm Hologon which is a true wideangle and doesn't protrude from the body a t all!

[Richard Knoppow]

The last first. The distance from the lens to the film plane is going to vary wi th the focal length. The very small distance for a conventional WA lens on a 35m m camera has to do with its very short FL; it's just going to be close. If you t ake 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 distan ce is going to be more with LF lenses. Actually, there are types of aerial camer a wide-angle lenses where the rear element is a field flattener element and is d esigned to fit right against the film, holding it flat also. There are both adva ntages and disadvantages to the "retro-focus" designs. Some aberrations are hard er to correct but some others get better. It is easier to design a retro lens wi th 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 compan y rep is touting the official company line. In general WA lenses are tough to de sign, especially if they are to have any speed. The very large front and rear el ements seen on many is to reduce mechanical vignetting as much as possible. I ho pe this is helpful and not even more confusing.

[Steve Pfaff]

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 im portant point. It is not. Some of the sharpest lenses are so called process le nses, 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 i n lens design necessitated by the presence of a mirror box on reflex cameras. T he focal length of a lens is measured from something called the rear nodal plan e, 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 t oward 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, th en the lens can (must) be further from the film. This allows space for, lets sa y, a mirror box. There are those who claim that a standard lens design offers b etter (sharper) lenses than either telephoto or retrofocus. Notice Rodenstock a dvertising these days that it is not necessary to buy telephoto lenses anymore a s 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 Biog on 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 t heir standard body. The Biogon lens couldnt be mounted to the standard body be cause 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 impor tant how far the lens is from the film. As to why LF photographers get such sha rp pictures, that probably has to do with the size on the film.