A black and white photograph “translates” the colours of the original scene into shades of grey. This “translation” or conversion is done by the film (in film photography), or by the sensor and software (in digital).
Some colour-to-B&W conversions seem simple enough. If the original scene has blacks, whites and greys, in a B&W photograph we would expect them to be rendered as blacks, whites and greys respectively. Likewise, we would expect a dark green to be rendered as dark grey, and a light green to be rendered as light grey.
What about a dark green and a dark red? That’s where things get more complicated. In a B&W photograph, we would expect them both to be rendered as shades of grey. But will they be the same shade of grey, or will one be darker than the other? The answer depends not just on the original shades of green and red, but also on the conversion process.
So how does this conversion process work? How can we control it? Why would we even want to? This is the first of two posts in which I try to answer some of these questions.