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Assumptions: 35mm SLR, 50mm "standard" lens, Ilford FP4 or similar film, capturing daylight scenes with normal colours.

What effects do colour filters have on film photos captured on black & white film? Are they used to balance/equalise tones in the resulting print, change contrast or do they have other applications?

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The following question/answer might be of help for context. For film this question/answer could also be helpful –  Peng Tuck Kwok Apr 5 '13 at 4:15
    
If you look at the answer by ex-ms to the question What types of filters are there and what's their use? he covers all the coloured filters. They selectively brighten/darken certain colours, for instance a red or orange filter will darken blues and give you more contrast in the sky. –  MikeW Apr 5 '13 at 4:17
    
The problem with the excellent answer by ex-ms at photo.stackexchange.com/questions/1872/… is that if I'm looking for information on filters specifically for B&W I might miss the entire question. –  Michael Clark Apr 5 '13 at 7:49
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marked as duplicate by MikeW, mattdm, Imre, jrista Apr 5 '13 at 23:49

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

1 Answer

Colour filter let's through the wavelength of the same colour that of the filter, and blocks other wavelengths. If, for example, you have a red filter in front of a lens, then a red rose will show up bright in the photo. In case of black and white film, the red filter passes the light from the rose through onto the film, which is then exposed well for the rose and will look white in the resulting photo. Everything else, if no other red subject around, will appear more or less dark (not black though), so the answer is: red filter increases contrast, but only between red and the rest.

Why is not all the rest just black, if the filter only lets red light go through? Because all colours are a mix of the three main colours, and have a greater or lesser amount of red component innit. Purple flower would show up in the red-filter photo quite nicely but the green grass in the background would appear very dark, as green is blocked by the red filter and never really reaches the film or digital sensor.

It does not really matter what camera you would be using, and if you want to experiment without camera, you can do it with a red lightbulb in a pitchblack dark room. The lightbulb should be of darkroom quality not to lett any unfiltered light out. Items in the room illuminated by the red lamp will appear the same way as in black and white film, although red, because we do see colours. Red items would look highlighted and the rest will be more or less dark. White reflect all wavelenghts and therefore white items in the room will appear about as bright as the red items.

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Thanks for spellcheck. I'm writing on a smartphone and the font is so small that I hardly see what I'm typing. And I forgot the white items showing up high. Often there's not many really white items in the usual outdoors scenery. –  Esa Paulasto Apr 5 '13 at 4:51
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