Archived posting to the Leica Users Group, 2003/05/17
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At 7:59 PM -0400 5/17/03, Don Dory wrote:
>Henning,
>I was mostly thinking of one of the aspheric elements forward or backward in
>space. Perhaps I am naive, but I think up to two stops vignetting in the
>corners doesn't bother most people. What was described was a severe hot
>spot in the center of the image. I don't think that even an unpainted
>portion of the lens barrel could cause a hot spot in the center; such a hot
>spot would move around the frame depending on where the source of light was
>coming from.
>
>One of my underlying assumptions in this whole discussion was that the
>original complaint was a real visible problem. Over the years, I have
>gradually realized that there is usually some basis of fact to reported
>problems. There is, of course, the occasional troll that throws gasoline on
>a lighted match. That is usually pretty rare.
>
>Don
>dorysrus@mindspring.com
Don,
The displacement of any element in _any_ direction will first be
noticed in a loss of resolution and an increase in aberrations. If
the displacement is axial, the effect will of course be symmetrical
while radial displacement will produce non-symmetrical deterioration.
Either type of displacement will produce very severe degradation well
before the slightest effect of increased vignetting is seen.
I don't think the post was a troll; rather a misunderstanding of the
design aim point and the resultant performance parameters.
I have noticed before that people coming from SLR's have different
expectations of optical performance than those used to RF cameras,
just as many cannot get used to the coincidence type rangefinders
coming from AF SLR's. It can be difficult getting used to the
different handling, just as the performance parameters of lenses of
different construction can seem weird.
My 21/2.8 ASPH, while having less vignetting (talking about optical
vignetting due to lens design rather than physical vignetting due to
mount or element diameter limitations) than the SA 21/3.4, still has
more light falloff than any recent good Nikon, Canon or whatever
20/2.8. It is noticeable if you look for it, or if you are sensitive
to it. Conversely, in all other respects it is a far superior lens
then those same Nikon, Canon or whatever.
The 21/2.8 ASPH is a very mild retrofocus design, constructed that
way so that metering is possible with the M5,6,7 cameras. This helps
the vignetting over that of the 21/3.4, but the strongly retrofocus
20/2.8 lenses for SLR's are better. There are a few tricks that are
used to expand the exit pupil size at larger angles so that light
falloff is less than demanded by cos^4, but at the present stage of
development, to maintain the best overall performance within a given
degree of retrofocus ratio will mean that there is more falloff with
RF wideangle lenses than SLR lenses.
The highest level of performance can still be achieved by using a
nearly symmetrical construction, and letting light falloff be as it
may. To compensate for this, a center filter is then used. This cures
all the problems except price, size and lens speed. Performance will
be stellar, as exhibited by modern LF designs.
You can't have it all.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com
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