John wrote:
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.
Strange that this effect is not more noticable in "normal" daylight shots,
especially with a lens like the 50mm f1.4.
Wayne Harridge
Ivanhoe, Victoria, Australia
http://members.optusnet.com.au/~w_harridge/
Exactly. One expects that effect, but its variability would indicate
that it is not the only explanation for darkened corners. Otherwise
you would not have lenses of the same focal length and angle where it
is pronounced like the 24/2 or less so like the 24/2.8.
--
Winsor Crosby
Long Beach, California
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