The conventions for standard focal lengths are more or less arbitrary. There's really no reason to keep to a specific focal length with an SLR, but then SLRs have only been dominant since the 1960s. Before that, focal lengths were locked in to certain conventional patterns because that's what rangefinder framing mechanisms were set for. Even now it doesn't make a lot of sense to make a 43mm or a 105mm for a Leica, because the cameras don't have framelines for those focal lengths.
For instance, nearly the entire reason why 135mm is a standard focal length is that it is the longest lens that can be focused using a rangefinder. When the migration to SLRs occurred, people wanted the focal lengths they were used to. So 135mm became a standard focal length for SLRs as well. Pentax is one of the few cameramakers who have consistently been adventuresome with focal lengths--with earlier lenses like the 30mm, 40mm, 120mm, and 150mm, and now with the Limited focal lengths. It's something I like and appreciate about Pentax, personally. There's lots about 35mm photography that is arbitrary. For instance, the size of the frame on 35mm movie film is 24mm x 18mm. Oskar Barnack simply doubled this to get the 35mm frame size of 24mm x 36mm. The 1:1.5 aspect ratio is too long. In the very early days of the Japanese camera industry, they took steps to correct this, by making cameras that had a frame size of 24mm x 32mm. But by then it was nonstandard and they were compelled to go along with the rest of the world. Similarly, the 50mm focal length is entirely arbitrary. The very first Leica, the Ur-Leica, had a shorter lens--42mm or something like that. This was discovered when the English Leica wizard Malcolm Taylor was asked to service the Ur-Leica by the company. But evidently 50mm lenses were easier to design (it has always been more difficult to design wide angles than telephotos) and it became the standard. It's just what Barnack and Leitz chose, is all. Then once it was chosen it became entrenched because cameras were made to frame that focal length. Other focal lengths were chosen based upon the 50mm length, based on the principle that a difference of less than 50% in focal length is not easy to perceive in pictures. That is, it's generally considered impossible to tell from looking at pictures whether they were taken with a 50mm or a 45mm, for instance. Thus, 24mm x 150% = 36mm (35), 35mm x 150% = 52.5 (50). Intermediate focal lengths between these were not considered necessary. The other influence on focal lengths pertained to what was most convenient optically. For instance, 28mm got to be a standard because it was the widest lens that could be designed in earlier times (28mm was once considered "superwide"). Early "superfast" normal lenses (as fast as f/1.4) were easier to design in a slightly longer focal length, around 55mm-58mm. So many early fast normals for SLRs are 58mm or so as opposed to 50mm. That may have been where 85mm lenses came from...those focal lengths times 150%; I don't know. As Mike Perham points out, the longer the focal lengths, the less difference a few millimeters makes. So nobody who shoots with a 400mm, for instance, feels a burning need to cover the 450mm focal length, whereas there is plenty of difference perceptually between a 20mm and a 24mm. None of this is really rational one way or the other. It's not like anybody ever sat down and said, "here's the best way to apportion focal lengths in a set of fixed lenses." Until, of course, Pentax did it. A set of lenses of 31mm, 43mm, and 77mm focal lengths is very well-considered, elegant, and useful, in my opinion. --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 .
