At 03:00 12/29/01, 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.
[snip]
Technical details: Kodak Max 400 (GC 400-6), Zuiko 24mm 2.8, with the
lens wide open, OM-1 shutter set at B with a locking cable release.
Exposure lengths were in minutes (as opposed to seconds.) The negatives
had sufficient density that the frame edges are fully defined.
Does the Zuiko 24mm 2.8 have a problem with light fall-off at the edges?
I vote for "cos^4 fall-off;" all lenses have some of this fall-off in the
corners. The shorter the lens the more pronounced the effect; the wider
the aperture the more pronounced the effect; and the some lens designs show
it more than others. "Vignetting" is a misnomer for the effect; it is
caused by light being spread out across a larger area of film in the
corners than near the middle. In addition, edge ray paths in the corners
travel through more glass subjecting them to greater absorption rates, and
can see the aperture at an angle making it effectively smaller (design
dependent). Stopping down the lens cuts off edge ray paths and reduces the
effect. You probably have some light pollution and/or ground haze, but it
appears to be second-order. It can amplify the falloff effect, especially
in the upper corners.
For a classic of fall-off with horizon lighting that exaggerates the
effect, see this one made with the 18/3.5 Zuiko:
http://johnlind.tripod.com/oly/gallery/om57.html
In this case lens and aperture were deliberately selected for the effect it
would have. Even though the notes state exposure data wasn't recorded, the
lens was wide open (f/3.5); it was shutter speed I could not remember.
-- John
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