How is it possible that I have gone more than a year without a single post dedicated to black and white conversion techniques? Black and white conversion is a large and sometimes complicated subject with many different approaches and differing opinions on which one is the “best” or most effective.
Since everything we do digitally is based in some part on how the medium's analog technology evolved over the years, we owe it to ourselves (and to everyone before us who made what we do possible) to have a basic understanding of that history and some of the key points in its development. How traditional color filtration works - A short history and science lesson
Why do it? Part of the history lesson…
Before panchromatic film (which was designed to be sensitive to all of the visual spectrum) there were silver collodion glass plates, which were sensitive to to about 400-520 nm, mostly sensitive to the higher ultraviolet and blue part of the spectrum, barely sensitive to the blue-green portion, and not at all sensitive to the green-yellow-red part of the spectrum. The photographs made with this process have a unique look, and usually have a pure white sky. There are some interesting compositional challenges because of this, which lead to the some of the first composites. Gustave Le Gray (btw, what a GREAT name for a photographer) would use underexposed sky from one negative and composite it into a print with a different negative—you try doing that without Photoshop…
When panchromatic films came around they were sensitive to the whole visual spectrum. This lead to some other problems with focus from chromatic aberrations and diffraction, but it also allowed for some interesting technical and creative controls. Photographers could now choose how much of the visual spectrum was used to expose the film. These creative controls are usually implemented to remove atmospheric haze, increase the separation of tones in skies (or make them really dramatic), increase the tonal separation or exaggerate the appearance of foliage, or separate tones based on color in rocks. Usually yellow, orange, red, or green filters were used, but a blue filter could be used to mimic the look of those 19th century photographs made with blue-sensitive films. Foliage can have a different and distinct look when photographed with a blue filter, which can be approximated digitally with color originals (because it is also possible to do this kind of black and white conversion with scanned color negative films and transparencies).
And now for the 10 cent science lesson
When working with traditional panchromatic black and white film you would often increase contrast in the scene by filtering certain wavelengths of light from exposing the film. This could be done with any number of color filters, for many different reasons, but they are all based on the same principle. There is a lot of math and physics involved in color science, and everything we do with digital cameras and computers owes existence to this field. Since I’m not a mathematician or color scientist I won’t go into any of that, but it is a good idea to have a basic understanding of how we see, how the camera/film sees, and how filtration works, so we can apply it to working with digital cameras and editing software.
All light starts as “pure” white, and is really just a combination of all parts of the electromagnetic spectrum, most of which we cannot even actually see. We “see” color as the part of the visible spectrum that is reflected back to us, and we don’t see the part of the spectrum that the material absorbs.
Panchromatic filters function the same way as any other object—additives of the glass absorb part of the spectrum and reflect back what is not absorbed. This becomes useful in black and white photography because the filters are made of optical glass, so the same colors that we see reflected back to us are also passing through to expose the film. The part of the spectrum we want to prevent from exposing the film is “stopped” before passing through the lens. In the next post I will go into how these kind of effects can be applied digitally from your color files.