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Re: [Rollei] OT: retrofocus & inverse square

- ----- Original Message ----- 
From: "john" <raga  >
To: <rollei  
Sent: Thursday, January 08, 2004 7:06 PM
Subject: [Rollei] OT: retrofocus & inverse square

> Heres a "curiosity-kills-cats" question for Richard...Do
> wideangle lenses suffer the same corner brightness dropoff
as a
> symmetrical design, or does their increased distance from
the film
> reduce the effect? If not, it would appear there is a free
> after all. My Nikkor 20mm seems to be reasonably even...no
need for a
> graduated filter. I don't have a Leica Super-Angulon type
lens to
> compare it with. -- John

   Emanual Biggler has given a very complete explanation. I
will add only a simplification. There are two contirbutors
to the fall off. One if the inverse square law applying to
light coming from the lens toward the film. The retrofocus
principle effectively narrows the angle of the exiting beam
and thus the variation of distance between center and corner
of the image, so the fall off from this factor is reduced.
This effect is greater as the retrofocus effect is greater
so it is more noticable for wide angle lenses than for
"normal" focal length lenses which generally have less of
the reverse telephoto effect.
   Another cause of fall off is the vignetting of the stop.
This can be easily seen by looking at a lens from an angle
from either side. The stop becomes cat's eye shaped. This
effect is also somewhat reduced by the retro focus effect,
at least on the exit pupil side.
   Some lenses employ a principle sometimes known as a
"tilting entrance pupil" to reduce fall off. This principle
is due to Micheal Roosinov, a Russian lens designer. It is
applicable to by symmetrical and asymmetrical lenses.
Technically, it is the introduction of some coma into the
stop (but not the image). Such lenses have stops which
appear to follow you when the lens is looked at from an
angle. Lenses based on Roosinov's design like the Super
Angulon and Rodenstock Grandagon, employ this principle to
some degree, some modern Japanese wide angle lenses have
almost no fall off from the pupil from this cause.
  All of these procedures carry some bagage in making other
corrections harder. For instance, symmetrical lenses are
automatically corrected for the lateral aberrations: coma,
lateral color, geometrical distortion. An asymmetrical
design requires that they be individually corrected. This is
one reason that symmetrical wide angle lenses have a
reputation for being better than the retrofocus wide angle
lenses for SLR cameras. They don't necessarily have to be
but the asymmetrical designs are harder to design up to a
given standard of performance.
   BTW, while the fall off of a "normal" lens is given as
cos^4 theta (theta being the image half-angle) some lenses
have more for various reasons. The famous Goerz Hypergon is
an example.
- ---
Richard Knoppow
Los Angeles, CA, USA