A low key image contains predominantly dark tones. The histogram of a low key image will be shifted to the left-hand side.
Pro tip
When photographing low key images, remember to underexpose the camera by dialling the exposure compensation to between -1/3 and -3 stops, depending on the circumstances. If you don’t compensate, the black tones will come out grey instead of black.
How you can take a similar image
Lens: Medium telephoto lens.
Settings: Medium aperture. Low ISO.
How: This image was taken in daylight, on a very misty day. I underexposed the camera to -3 stops to make the whole image dark and to create a moody effect.
This tip is from the following book: