[Leica] Leica-Users List Digest V2 #139

[Mike Johnston <70007.3477@compuserve.com>]

>>>I have been trying to follow this thread hoping that ultimately I will be
educated as to what is and what is not a retro focus lens....Sorry to hold the
class back, but I'm lost<<<

 Damian,
 "Retrofocus" is a colloquial name for an inverted telephoto type lens; the
word was originally a trade name of the French Angenieux Co. and in time has
become generic. Inverted telephoto types are lenses in which the second
principal point is behind the lens itself, so that the back focus is longer
than the focal length. (Back focus is the distance from the rearmost glass
surface to the film place when the lens is focused at infinity, i.e., when it's
as close as it ever gets to the film plane.) Inverted telephoto types are used
in SLRs when the focal length of a symmetrical lens creates a back focus
distance that shorter than the distance the camera's mirror needs in order to
flip up and get out of the way. Usually, a negative lens assembly is placed in
front of the main lens assembly, meaning that retrofocus lenses are typically
larger and more complicated than symmetrical lenses of the same focal length.

 --Mike