I recall, some many tears ago, seeing a typically well crafted Nikon lens which had a dramatically short focal length. These craftsmen at Nikon had devised a way of placing the lens further from the film, making the lens seem to be a longer focal length. I believe it was a retro lens or somethng. Leica never did do the same thing, and I wonder why. I can only presume it was a quality issue.
With SLR cameras in particular it is only possible to achieve short focal
length lenses that cover the full 24x36mm frame (assuming the mirror will not
be raised in order to install the lens and thus negate TTL use) by relying on
an approach that is called retrofocus design.
This design starts with a negative lens out front whose function is to make a right-side-up virtual image in space out in front of the lens. Then, a positive lens group is used to focus or reproduce this image at the film plane of the camera. By judicious choice of focal lengths of the negative and positive lens groups it is possible to make a short focal length lens have a larger glass elements to image plane distance than a simpler lens made up only of a single positive lens of the same focal length.
In my convoluted way I am trying to say that a simple lens of 24mm focal length
is located 24mm from the image plane when the lens is focused at infinity. A
24mm focal length lens of retrofocus design is made up of a negative and
positive lens working in combination and in which the distance from the rear, positive, lens to the image plane might be 50mm, giving "room" to place a
mirror that requires a given amount of space (maybe 40mm) between the rear
element and the image or film plane.
With rangefinder-type cameras such an approach is not normally required as one
does not need to be concerned with a mirror between the lens and the film plane
and so the retrofocus design is hardly ever used for lenses designed for such
cameras. Maybe it never was but am not positive about this. The reason is that it is significantly more expensive to go the retrofocus route to build a short
focal length wide angle lens.
Leica did indeed make retrofocus wide angle lenses like everyone else. Any wide
angle lens for the Leica reflex is a retrofocus design. Somebody, maybe Nikon,
did, however, make a short focal length lens that required the locking-up of
the mirror to allow the attachment of the lens to the body - meaning the SLR
became a camera that required an auxiliary optical finder for use. But my
memory may be faulty on this.
BTW, telephoto design is a related approach in that it is also a "combination" lens but in this case the desie is to make a lens whose physical dimension from the image plane (with lens focused at infinity) to farthest lens element is shorter than that of an equivalent single element lens. Here the front element or group is positive and the rear group is negative. The function of the rear negative lens(es) is to make it appear that the light rays are coming from a location father away than the location of the front, positive, lens or group.
This is also the principle behind "tele extenders" or in astronomy Barlow lenses. Optical "magic" of sorts!
Afocal lens attachments that change focal length are another interesting optical sleight-of-hand in that they provide to the camera lens an image that appears to originate at infinity. Got to think about that ... and I apologize if I've made any major errors above. Little ones I make all the time I know! :)
Well, I guess it is time for coffee again and for another breakfast.
andy
Andrew Davidhazy, School of Photographic Arts and Sciences/RIT
andpph@xxxxxxx http://www.rit.edu/~andpph
