Re: [Leica] Retrofocus wide angle lenses

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

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

There are generally only two reasons why a retrofocus construction is
considered: 1) to get the rear elements of the lens far enough away from
the film plane to allow a reflex mirror to be inserted or TTL metering to
take place or 2) to allow the optical designers to take advantage of the
characteristics of retrofocus lenses. These are an inherently better
eveneness of illumination due to the rear exit pupil being further from the
film plane, so that the exit angle is not as sharp and the cos^4 law
doesn't reach as small a value, and to create faster wideangle lenses,
which is somewhat easier with retrofocus lenses due to a number of factors.

Leica, in the design of the 21, 24 and 28 lenses used all these benefits to
some degree. The first retrofocus lens of the bunch, namely the second
version of the 28/2.8, came about due to the need to get the rear element
far enough from the film plane to allow the metering cell of the M5 to get
behind it. It also allowed the designers to correct the lens a bit better
at the 2.8 speed, and it automatically gave better illumination.

I've never looked closely at a Russar 5.6, but I can imagine that the lens
elements are very tiny, and the total length of the lens cells (front of
front element to back of back element) is probably not more that about
1.5cm, so that metering can take place even though the lens is not a
retrofocus design, but faster lenses usually need a longer cell length,
making a retrofocus design necessary. The one penalty that still seems
inevitable with retrofocus lenses is that they display complex distortions,
even though the Leica M lenses, being only slightly retrofocus and designed
to extremely high standards, exhibit about as little of the typical
retrofocus distortion as any lenses of their respective focal lengths. They
are just not as good as symmetrical lenses (such as the 21/3.4)

   *            Henning J. Wulff
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