Archived posting to the Leica Users Group, 2002/02/18
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>Hi everyone,
>I just test a roll of film on my leica M6 TTL with Kobalux 21mm lens. I
>check the neg. and it look like alot of light fall off at the edge of neg.
>both side of negative not top and bottom. does anyone has this problem ? I
>check with kobalux website and also picture at Dante Stella website and
>those pictures did not seem to have light fall off. these pictures might be
>crop out .
>can someone who use kobalux please confirm this matter for me. is this
>light fall off usually happen to wide angle lens such as 24mm down?
>Olivier
>
>
>
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Almost all lenses produce falloff in the corners; 21mm non-retrofocus
lenses of modern design typically show about 2 stops falloff in the
extreme corners; this is about what the 21ASPH produces. The C-V 21
has slightly more. A good retrofocus SLR lens has about 1 to 1-1/2
stop falloff.
This has to be considered in view of the fact that the Noctilux has
about 3 stops falloff (all wide open) and most telephoto lenses have
about 1 stop falloff, with many zooms having 2 stops falloff.
Stopping down the lens gets rid of the falloff due to the mounting of
the lens elements, but not the falloff inherent in the optical design.
Falloff does not change due to sample variation (quality control) but
is inherent in the design. If one Kobalux lens has a certain amount
of falloff at a certain aperture and focussing distance, another of
the same design will have the same amount. Some films (such as slides
or high contrast B&W) will accentuate falloff, and some (such as
color negative or chromogenic B&W) will mitigate the effects.
To check at what apertures the vignetting due to the mount design
goes away, or whether filters and shades cause vignetting, check in
the archives. I've written on this topic quite a few times in the
past.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com
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