Originally posted by Blue Where the confusion comes about is the interchangeable use of Normal & Standard lens. The standard lens is defined by the film or sensor diagonal. Additionally, this doesn't apply solely to 135 slr cameras. The definition of a standard lens is one where the focal length of the lens roughly equals the diagonal of the image area (film, plate, sensor etc.). It doesn't stipulate it has to be for 135 (aka 35mm). John Hedgecoe has referred to a normal lens as a standard lens.
Old engineering saying: The nice thing about standards is, there are so many of them.
As noted somewhere, 35mm was the standard for Leica RF's, 40-45mm the standard for many fixed-lens RF's but some went wider, and other 135/FF cams had whatever standard the designers wanted. Like legality, standard means whatever you can get away with. I think the standard for any format's SLR's was a bit longer that that for RF's and non-interchangeable-lens cams. Yah, blame the mirror. When FF or APS-C EVIL's finally escape into the wild, their standard will probably be a bit wider than for their SLR counterparts.