Removing color casts from wide angle lenses

Do movements shift the falloffs?

Alan Edward Klein said:

Which lenses would you avoid using polarizers in 4x5? Does that apply to both BW and chromes?

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The effect that a polarizer has on the image varies with the relationship between the angle of view and the direction of the light - the corners will always be different than the centre. The visibility of that change in effect will be greater with wider lenses.

Angle of light with polarizer is only one part of this.
Natural polarization is second part. Polarization occurs naturally as light passes through air. Using polarizer outdoors with wide angle lens is like using variable ND filter (consists of two linear polarizing filters) that is also variable across the frame.
It's weakest at 0 and 180 degrees - shooting directly into the light source (in this case sun) and with sun behind the back.
It's strongest at 90 and 270 degrees in relation to light source.

Quick search for "polarization of sky with wide angle lenses" gives following first three hits with some sample images and processing techniques (don't have any relation to these sites):
http://www.earthboundlight.com/phototips/wide-angle-uneven-polarization.html
https://www.photocascadia.com/how-to-fix-dreaded-polarizer-sky/
https://paraselene.de/cgi/bin?_SID=..._sprache=paraselene_englisch&idartikel=113902

If angle of view is 40 degrees lens is only seeing 20 degrees left and right if you're shooting into light source. If lens is seeing 50 degrees each way effect is significant.
For the same reason, when shooting reflective objects in studio, light needs to be polarized to remove reflections.

This is the same reason why correction file is different with different light sources.

itsdoable said:

1/cosine4 is a geometric effect, the circular aperture appears smaller (oval) when you view it from an angle. You see this particularly on pinhole images. It does not affect colour cast.



These make sense to me, vignetting increases the saturation in the corners, which may accentuate an over all magenta cast.

Also, most neutral density filters I've used have a slight cast, if the center filter has a slight green cast, the corners (which have no density attenuation) would not have that cast, so when the image is colour corrected, the corner will go magenta.


PS: took me a while to re-find this thread after the move...!

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It does not affect colour cast.===> It does when a uniform filter is added, especially in a very wide lens. That is the case here. Then the removal of that cast is nonlinear and becomes difficult. That, I believe is the problem.

Sirius Glass said:

It does not affect colour cast.===> It does when a uniform filter is added, especially in a very wide lens. That is the case here. Then the removal of that cast is nonlinear and becomes difficult. That, I believe is the problem.

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This explanation does not make sense to me, the peripheral rays have a longer path through the glass filter, which results a change in field curvature and the addition of Lateral CA. A uniform, clear or neutral density filter does not change colour. Perhaps the introduction of red lateral CA which is not resolved by the finite thickness film layer will cause a magenta cast?

LCC on C1

buy all kinds of CF