At 10:19 3/6/00 , Ángel Lobo wrote:
>After get some vigneting with normal pol. filter on 24 and 21mm, last Febr
>I buy one circ. pol. filter of B+W on the thin (and expensive) version.
>
>I take a test picture with the Zuiko 21mm f 3.5 on f 8 to a clear blue sky
>and I have a great vigneting.
>
>This new filter is " B + W 49ES SLIM C - POL Germany ".
>
>Some experience with this Slim B+W filter ?
>
>Can someone tell me one Circ. Pol. for the 21 3.5 without vigneting ?
Ángel,
Are you sure you are not seeing "cos^4(theta)" falloff (where theta is the
angle from lens axis)? Possibly compounded by the polarizing filter, but
not vignetting from the filter ring?
Cos^4 falloff is caused by two factors:
1. Light off-axis to the lens will see the aperture turned slightly making
the aperture hole an ellipse, not a circle as seen by light on-axis. This
reduces the amount of light from the edges, and the corners in particular.
This contributes a cos(theta) rate of falloff.
2. A rectilinear lens (the 21mm is rectilinear) preserves angles, not
areas, in mapping a plane in space to the plane of film in the camera.
Areas of objects increase from center to the edges and especially the
corners. This means the light is spread over a larger patch of film from
the edge and corner than from the middle. This contributes a cos^3(theta)
falloff. (Note: a spherical "fisheye" lens maps spheres in space to the
film plane and preserves areas, but not angles.)
Lens designers can compensate for both in most lenses, but the
"super-wides" are the most difficult and some falloff is to be expected.
One way to look for filter vignetting (and lens hood vignetting too):
1. Stop the lens wide open.
2. Place the camera shutter speed in "B" and lock the shutter open with a
locking cable release.
3. Open the back and carefully look through the film gate and wide open
diaphragm, especially at the corners. If you line up the corner and edges
of the film gate with the diaphragm you should not see any blockage from
the lens ring on the front of the lens (or in this case the filter ring).
Any blockage is likely vignetting. The amount of vignetting (severity) can
be estimated by how much of the aperture area is blocked.
4. If any vignetting is seen, then gradually stop down until it is gone.
This is the widest aperture without vignetting and will also give you an
idea of how severe it is.
Hope this helps,
-- John
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