This week's Photography Fundamentals issue looks at key. Key is an element of the photography canon that crosses over with other artistic disciplines, most notably music and painting. I'm the least musically-talented person known to man, but even I manage to spot the similarities.
When we talk about the 'key' of an image we're talking about the range of tones or brightness that it comprises. Primarily we use it when we're describing images as being either 'high-key' or 'low-key', which are at the extremes of the range of brightness—light or dark respectively—and the feelings that these images convey. However, 'high-key' and 'low-key' can also be used to describe lighting set-ups, not just a style of photo.
High-key
High-key images are light and bright, either with upbeat and positive connotations or with dream-like, ethereal qualities. They will be low on contrast with very few, if no, shadows. If you look at a high-key image's histogram, it will exist mostly in the right half of the graph, with just about all of its pixels pushed above middle-grey and into the near-whites and whites.
The intensity of colours begins to fade as brightness increases, which means that high-key images are frequently black and white. If they are in colour, they tend towards pastels in tone. Or they could be the classic white-on-white.
It's easy to think of high-images as being 'just over-exposed', but getting them right is a bit more complicated than simply setting some positive exposure compensation. To achieve a good high-key image you need to bathe your subject in even light and keep everything about the image on the pale side. Unless you have deliberately blown-out the background to get it bright white, and with the exception of specular highlights, there will still be detail across the image.
I like to think of high-key images as the photographic equivalent of reading Jane Austen, but you can pick your own literary metaphor. Music-wise, it'd be a song composed in the major key.
Low-key
Low-key images evoke feelings that might be sombre or miserable, or even fearful or threatening. Like high-key images, they're low on contrast, but this time they are predominantly dark or black in tone and their histograms are clustered towards the left-hand side of the graph.
Anything that needs to be portrayed with a sense of impending doom is perfect low-key material. Just as high-key images aren't all about over-exposure, low-key images aren't focused on grisly under-exposure. There will still be detail in the shadows. If you don't want to be too purist about how your histograms look, having the odd bright area can strenghten the feel of a low-key image by re-inforcing just how dark the shadows are.
If you want a literary comparison, think gothic horror novels, or the minor-key for a musical equivalent.
High-key and low-key lighting
Cinematically, high-key or low-key lighting means something quite specific. High-key lighting has a low key-light to fill-light ratio that produces evenly lit scenes that are practically shadow-free. Low-key lighting, on the other hand, has a high key-light to fill-light ratio (yes, it's counter-intuitve) that creates pools of light and harsh shadows.
TL;DR
- Key refers to overall tone of an image
- High-key images are light and bright with a general sense of positivity
- Low-key images are dark, brooding, and can even feel menacing
- Although high-key and low-key images rely on technical over- or under-exposure to achieve them, this is controlled and does not negate details in the highlights or shadows respectively
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