So. You’re taking a picture of a guy who’s wearing a white shirt and black slacks, and you’re using 400TX or some other film you intend to develop yourself. Typically, you want the most information recorded on the film that you can get, so you’d like to maximize the amount of detail toward both ends of the tonal spectrum (assuming that the midtones are assured, by careful processing). Here’s a strategy that’s better than win/win, if you can believe that.
Set the meter on your camera to 200–half the published ISO; now we call it our Exposure Index, or EI.* This tells the meter that the film is half as sensitive as it really is, so that the meter will recommend a combination of shutter speed and aperture that allows twice as much light onto the film as it “needs.” Now the baggy wrinkles in our model’s (we’ll call him “D”) trousers are recorded more fully on the film. But wait: what about the folds in the shirt? Won’t the white get blown out by overexposure? Au contraire, mon frere. We can use a new, shorter developing time to keep that from happening. Less time in, say Xtol means less tendency for the grains to clump, resulting in slightly less apparent grain; it’s win/win/win!
Here’s a more technical, very clear explanation of the above:
http://www.youtube.com/watch?v=rlnt5yFArWo&feature=related
…and here is a less technical, very unclear version:
http://www.youtube.com/watch?v=VGBXHCGFOgw
*Alternately, for some cameras, set the exposure compensation function to +1 (but never both at the same time).
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