Tri-colour exposures

May 8, 2009

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This is a colour photography process I’ve been exploring off and on for the past couple of years. Essentially it is making colour photos using black & white film. It’s an interesting way to learn how colour images are constructed in both film & digital photography. It is basically the same process used by early colour experimenters such as James Clerk Maxwell and is based on the understanding that the primary colours of light are RGB; red, green and blue, and that filters of these colours can pass the wavelengths that match and block those that don’t. The simple example is that a red filter allows the red light (wavelengths) to pass through while absorbing (blocking) the blue.

Hence this process is known as tri-colour as we have to make three exposures to record the full range of light. One exposure through a red filter, one through green and one through a blue filter. So we have 3 B&W negs that are colour separations of the whole & need to be re-combined to make a colour photo. There are some gum printing processes where this can be done direct to paper, however the most common method & the one I’m displaying here is to scan the film & combine the colour layers in Photoshop.

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detail of my room in Hotel Roble, Mexico City. I liked the peacock tail effect the maids would make with the toilet paper roll.

The first step is to get some filters that will go in front of the lens for each shot. I use a 1920s Voigtlander Avus, for various reasons but one of them is that it has a small lens diameter (approx 26mm) so only needs small filters. It also has an extending lens bed so rather than having to source filters to fit over the lens I could make them in a cardboard mount that sits on the lens bed, as long as I’m shooting vertical frames. The filter sizes are so small that I was able to get them for free from a Lee filters sample swatch book that was given to me by Panavision. The next step is to establish the filter factor for each filter, i.e. how much light does it block that needs to be compensated for to obtain correct exposure. I metered through each filter with a spot meter and found that the Red filter needs 3 stops extra light, the Green requires 4 extra and the Blue gets plus 5 stops.

To make a photo I set the camera on a tripod, get the framing right & do some basic metering as if I was exposing B&W film normally. The Voigtlander was made to shoot 6.5 x 9 cm glass plates but mine also came with a 120 film back that shoots 6 x 9 cm frames. I always expose in RGB order & adjust the basic meter reading by the filter factor. For example if the normal exposure would be f/8 at 1/60 sec, I would expose R at 1/8, the G at 1/4 and the B at 1/2 sec. 6×9 on 120 gets me 8 frames so I generally shoot one frame without the filters at the base exposure as a reference.

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portrait of Steve

The process described above is exactly how regular colour film works except there the 3 layers are exposed simultaneously and in perfect registration. With tri-colour the exposures are sequential so if something in the scene moves betwen frames it will result in a colour shift when they are combined. This has interesting implications for subjects like portraiture and is an area I intend to explore further.

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