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Re: [Rollei] OT: retrofocus & inverse square



Richard

You mentioned Goerz Hypergon so I looked it up - what excactly did the 
"star" do
The Goerz is mentioned as something very special for Large Format - 
where they rare or common among prof. photographers

Regards
Ruben

Richard Knoppow wrote:

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