Quote Originally Posted by Nikon Collector View Post
Interesting, I always thought the focal length was the distance from the middle of the lens to the film plane, didn't know it was variable
The problem arose with large format cameras when a long (sic focus) lens was used and needed triple extension racks and bellows, the wind resistance ment the tripod could need guys.

So the optical people put a negative (concave) lens behind the long focus lens to telescope the bellows.

Today the design computers agonize about optimising the whole design but the rear of a telephoto lens is still essentially negative.

The SLR wides and even many of the Leica wides are inverted telephoto lenses which are longer (further away) from the film plane than a simple short focus lens.

Some of the pancake lenses are 'squeezed' optically.

Any of these unnatural lenses are more complex than they would otherwise have been, more $...