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Coloured lens filters for control of relative exposure of colours within a shot?


James Foster

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[Question is for a Super8 film but should be broadly applicable]

Coloured lens filters can be used to adjust colour cast, however, as with the use of colour filters with black and white photography, it is my understanding that colour filters also effect the relative exposure of different colours, eg. red filter raises the relative exposure of red in the image. Would it therefore be considered possible within reason to apply a subtle coloured filter to the lens and then in post production to correct for this colour cast, thereby adjusting the exposure of the relevant colour (for example can warming filter be used to darken a blue sky while the warming effect is negated in post-production?)? Would this be problematic/ to the detriment of the image in other ways?

-James

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Yes, you can do that, and yes, that's the effect it will have.

It's something that's worth testing because the various colour processing systems - the sensor, the camera electronics, the grading software - have various nonlinearities and mismatches which mean this sort of thing does not always work in a straightforward and predictable way.

P

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Filters filter, they don’t raise exposure, they lower exposure. A red filter cuts more blue and green wavelengths relative to red but it doesn’t increase red exposure.

This selective color contrast manipulation using color filters only really works for b&w panchromatic films. If you use color films on color film, you create a color bias but once you correct it, you don’t really get a different contrast in each color layer in RGB printing (of course with digital color-correction you can play with the gamma and luminance of selective colors.)

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Just playing with a deep orange lighting gel in front of the lens, I just shot this test. Now I can't correct color negative to color print using RGB printer lights, this is a digital photo.

So here is the image shot with and without the orange gel:

 

orange_filter3.jpg

orange_filter4.jpg

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This is a comparison between correcting out the orange cast just by shifting the color temp to blue/lower (which makes the image very green) and shifting the green to magenta -- compared to taking the normal color photo and just lowering the luminance of the blue channel:

orange_filter2.jpg

orange_filter1.jpg

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Thankyou both for your very considered replies, I think I will shoot a couple of stills on the Super8 as above to see what it's like to play with but I expect I will get a similar result to yours David

 

Thanks again,

J

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