Re: [Leica] Re: f 1.0/21 ... 1.0/35

["Henning J. Wulff" <henningw@archiphoto.com>]

Dan Post wrote:

>My understanding was that retro-focus lenses were the result of the SLR- the
>rear element of really wide lenses interfered with the mirror; since the
>element is further away from the film, the angle of which the COS affects is
>smaller. Were or are there any retrofocus LTM or M lenses, and if so was the
>design used to eliminate any fall off on the edges of the field?
>
>
>>In optics these are part of the 'cos to the fourth' function that standard
>>construction lenses obey in a general way. With a bit of fudging this can
>>be compensated for to a small degree. If you want a lot better eveness of
>>illumination, retrofocus lenses are the ticket. You get a lot of other
>>problems, but eveness of illumination can be greatly improved.
>>
>>Telephoto designs make matters worse, by causing more falloff than the
>>'cos-fourth' law dictates, but it usually doesn't matter much because
>>telephoto designs are used on long focal lengths, where the 'cos-fourth'
>>falloff is small anyways.


You are quite right; the retrofocus design was mainly developed to allow
wideangle lenses to clear the SLR mirror. It just gave the added benefit of
reducing the 'cos-fourth' falloff.

The only 35mm lens I know of that was specifically designed to have no
light falloff was an extreme retrofocus design that also employed a
strongly aspheric element, namely the 10mm/5.6 OP fisheye made be Nikon in
the 60's.

It's debatable at exactly what point a lens is no longer a 'standard'
design, but a 'retrofocus' or a 'telephoto'. The usual definition of a
retrofocus lens is that the rear element when the lens is focussed at
infinity is further away than the focal length of the lens. I don't think
any M lenses are retrofocus by this strict definition. On the other hand,
if the general design priciples of retrofocus lenses are used to move the
rear lens element away from the film plane, but not enough to qualify by
the above definition, then it is still considered a retrofocus lens by most
people. In this case we have some winners.

At the moment there are 3 M lenses that are true retrofocus designs,
although hardly extreme. They are the 21/2.8 ASPH, the 24/2.8 ASPH and the
28/2.8. All 28/2.8 lenses except the first have been retrofocus, and the
non-ASPH 21/2.8 is also. In every case this was done to allow metering with
the M5 and M6 cameras. They do have slightly less falloff than their
non-retrofocus counterparts, but since they are only slightly retrofocus,
their additional light in the corners is very slight as well. I'm not much
of a 28 user, so I haven't used all the 28's, but I believe the present 28
is close to if not better than the first 28/2.8 (non-retrofocus) design.
With respect to the 21's, the old 21/3.4 SA (non-retrofocus) is still
better than the newest 21/2.8 ASPH, so going to a retrofocus design purely
for better illumination is usually not advisable.

The old Summilux 35 had a design that increased illumination in the
corners, but it was not a retrofocus design. There were no LTM lenses made
by Leitz that were retrofocus.


   *            Henning J. Wulff
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