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| p.3 #3 · 5dmk2, problems with pure reds (and sometimes blues) |
It works exactly as advertised. The filter adjusts the raw exposure levels of the channels, which can't duplicated in post by non-optical methods (such as lowering the overall exposure). If the only hue in the image is red then it wont matter, but every rose has its non-red thorn, not to mention a nice blue sky if the composition warrants it
If the only hue is red, than all the filter does is... ta da!... reduce exposure... just like using a smaller aperture or a shorter exposure.
If there are other hues, the balance among them will become unbalanced if the filter issued, and you will have to compensate (un-compensate?) in post to bring them back in balance again... thus effectively lowering their luminosity levels. Just as would have been the case had you used a smaller aperture, faster shutter speed.
Play it out with an actual exposure and I think you'll see what I mean.
Comparing different lenses on the same body, I have seen color balance shifts up to 1/2 stop (and 1/4 stop level variations are not uncommon). This can be enough to make the difference between clipping or not if you are shooting right on the edge of color saturation. There could even be some truth to generalizing this to whole brands, since manufacturers often strive for a consistent color balance across their product lines; however, it really needs to be evaluated on a lens-by-lens basis.
Several comments on this notion... First, I'm not quite sure what a "1/2 stop... color balance shift" actually means? Are you thinking of simple light transmission variations? That can happen, but that is a different thing than color balance, and it would still be a matter of off-setting the exposure to compensate for the hot color channel... and since the metering happens through the lens, in the end it would not have an effect on the amount of compensation required relative to the "normal" exposure using that lens.
And let's say that something you describe as a "1/2 stop shift in color balance" can be found between the two worst outlier lenses, and more typically the "shift" is less than "1/4 stop." By this description I presume you mean something like (to make up an example) that some lens reduces the luminosity of some color (that does not necessarily correlate precisely to a color channel on the sensor) by as much as "1/2 stop" compared to some other color. I'll make another wild guess that here in this "worst case" that the hotter color (let's say blue) might be 1/4 stop hotter than it should be and the less hot color (let's say red) might be a 1/4 stop less. This hypothetical difference is truly tiny and would rarely make a difference at all. I suppose that if you had some very faint detail just barely below the very brightest level in the hot color channel and you pushed the exposure right to and just barely past the "edge" that you might see some slight reduction in the contrast between these details. But that's about it.
(And notte that the OP came back and mentioned - a few posts above - a much larger compensation required to fix the problems with his red images.)
Two more points. First, the whole "lens coloration" thing tends to be grossly overblown by folks, many of whom buy into the mysticism of lenses with "wonderful color" and the like. Whatever color shifts lenses impart to images, generally I don't want them, but fortunately they are always much, much smaller than the color shifts we deal with based on changes in light coloration in our photography... and they are quickly and easily (in most cases) fixed in post.
Second, once again we are heading off on a tangent that has little to do with the real and practical solution to the OP's issue, and instead wandering off into Theoretical Land because, well, why exactly? As the OP reported a few posts up on this page... decreasing exposure in the images with exceptionally strong red fixed the problem. Simple. Easy. It works.