On Fri, Dec 28, 2001 at 10:00:37PM -0500, Larry Woods wrote:
> I have a question about night-sky pictures that I took back when the
> Leonid asteroids were around. All of my pictures have a "hot spot"
> more-or-less in the center of the frame.
>>
> Does the Zuiko 24mm 2.8 have a problem with light fall-off at the edges?
>
Hi,
I´d say it is the combination of light-fall-off at the edges of the lens
(vingetting)
and the uneven light in the sky:
cos(w)^4 is nice, a spreadsheed, and here are the
values for the natural vingetting:
focal angle of light
lenght field loss (f-stops)
------- -------- --------------
18mm 100° 2.5
21mm 91° 2.0
24mm 83° 1.6
28mm 75° 1.3
35mm 63° 0.9
50mm 46° 0.5
100mm 24° 0.1
It is obvious that natural vingetting is a concern of
wide angel lenses. But this values are not all,
which is demonstrated from the following values from a lens-test of
two -here- well known lenses:
wide open F5.6 F8
Zuiko 3.5/18: 2.6 1.7 1.3 [F-Stops]
Zuiko 2/21 : 2.8 1.5 1.2 [F-Stops]
There are two other factors: Imperfect lens mount cut additional
light from the rays of the corners. Happens when the diameter of the lens
elements are not wide enough (called: factitious-vingetting??).
This effect is reduced when you stop a lens down (see above).
The other factor is "RETROFOCUS DESIGN!". This enlarges the
lens-to-film distance. This effects also that the light strikes the film in
a less "troublesome" angele and vingetting is reduced!
Concluson: SLR-wide-angle lenses are more complicated to built,
Compared to classic, symetric w.a.-lenses for rangefinder and large format.
But this helps to prevent vingetting.
Frieder Faig.
P.S: The XA lens (2.8/35) uses a tele construction, to make the lens
shorter ("Inverse of Retrofocus design"). This is why the lens is more prone to
vingetting than the theoretical value for 35mm lenses- What Oly. admit in
the Vision-Age-Magazine.
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