Re: [Leica] Retrofocus wide angle lenses

["Ken Iisaka" <kiisaka@ibm.net>]

From: Doug Richardson <doug@meditor.demon.co.uk>
> Marc James Small <msmall@roanoke.infi.net> wrote:
>
> >Erwin Puts and I have had some lengthy discussions about these [21,
> 24 & 28mm] designs, but I still don't know why Leica chose to use
> retrofocus designs for them.
>
> The elements of a retrofocus design are larger than those on a
> conventional wide-angle, so are easier to manufacture and mount. I'm
> always surprised by the difference in size between my 20mm f5.6 Russar
> and my 21mm f4 Super Angulon-R. The inner groups of the Russian lens
> must be very small.

Actually, while the front elements of Retrofocus (tm) are big, inner
elements certainly are not.  Some elements in the huge, 13mm 1:5.6 lens by
an infamous Japanese manufacturer are merely a few millimetres in diameter,
no bigger than the middle element of the original Hologon.  One of the many
obvious reasons why Leitz opted for Retrofocus (tm) design was to provide
the clearence for metering.  This is still true in case of M6.

> Perhaps Leitz thought in neccessary to move to retrofocus in order to
> get the speed of 28mm lenses to f2.8.

The first generation Elmarits were not Retrofocus (tm).  However, it is true
that there are more implementations of Retrofocus design in fast wide angle
lenses.  The common symmetric wide angle configurations such as Orthometar,
Biogon (which is essentially two Retrofocus lenses stuck back to back,)
Topogon, do not lend themselves well to bright interchangeable lenses
partially because the diameter of the rear elements would be limited by the
diameter of the lens mount and the amount of space behind the mount, and
partially because the correction of the light fall off would be extremely
difficult, necessiating huge, negative meniscii elements front and back.