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.
>
> 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.
>
With that lens I'd expect that amount of visable fall off.
Wayne Harridge
Ivanhoe, Victoria, Australia
http://members.optusnet.com.au/~w_harridge/
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