Originally posted by Imageman This has I think to do with convention and to space available for the mirror box in an SLR.
The focal length is the distance between the rear element and the plane of focus. It was usual to not employ a retrofocus design if possible, (a retrofocus design is a design that keeps the rear element away from the camera body by using extra elements) because with 50mm and longer lenses there was more than enough space to stop the mirror hitting the lens as it lifted,
Focal length is measured from the principal plane (H') of the lens (often called the optical center) to the primary focal point or plane. A single thin lens could be placed at H' to achieve the same focal length but the light would focus to a point.
Volume 1, Chapter 33. The Human Eye as an Optical System
Consider this diagram - H prime (H') is
before the front element.
Originally posted by Imageman
Now lets consider pentax.
By convention, the 50mm would be a good focal length photographically, however lenses at that focal length bring the rear element so close to the mirror box that conflicts can happen. The 50mm f1.4 for example is unusable on some bodies due to the mirror striking it. The 55mm f1.8 has no conflicts with the mirror on any camera that I know of.
I have heard that the 50mm f1.4 did in fact hit the mirror in at least one pentax model.
There is no significant difference between 50mm 55mm or indeed 60mm, so if a 50mm is a bit too close to the mirror box it makes good business sense to use 55mm in your design rather than the almost identical 50mm focal length.
With the 50mm f1.4 Pentax chose it seems to live with the mirror conflict possibility at this shorter focal length, rather than go the more expensive route of retrofocus design with extra elements.
The registration distance for Pentax bayonet (K) and screw (M42) mount is 45.46mm.The registration distance is measured from the base of the flange on the lens (or top of the mount on the camera body). That would mean the rear element on a 50mm need only be 4.54mm
away from the body or 4.54mm into the lens body.
If focal length is always measured from the rear element then there would have been no need for anything on a 50mm lens to protrude past the flange other than the aperture linkages. It would also mean that telephoto lenses would have a tube the equal to the focal length less 45.46mm. It would mean zoom lenses would always have floating rear-element that travels the range of the zoom less 45.46mm (not all zooms have floating rear elements). It would mean wide-angle lenses rear-elements would have to protrude into the camera body by 45.46mm less the focal length of the lens. Even with retrofocus designs if focal length is measured from the rear element then the K-mount would be limited to say no wider than 30mm.