Exposure meters are a photographer’s friend. After all they help you get pleasing photos, time after time. However, they’re not perfect. Exposure meters work on the assumption that the scene you’re metering has an average tonal range, equating to a mid-grey. Generally this assumption is a fair one – most scenes you’ll shoot will have a tonal range that does not deviate much from this ideal.
But (and you knew there’d be a but didn’t you?) exposure meters do get fooled. There will be times when your photos are oddly under or overexposed. This is generally due to the tonal range of the scene either being darker than average (which tends to cause overexposure), or lighter than average (causing underexposure). The solution is simple. For the first situation, you apply negative exposure compensation to make the image darker and ‘correct’, or, for the second, apply positive exposure compensation to lighten the image.
But hold on a minute. Perhaps, for the simple reason that it produces a more pleasing result, you actually want your photo to be brighter or darker than average. In fact there are terms for these two effects. A photo largely made up of dark tones is said to be low key. In a low key image shadows are generally dense, with little or no discernible detail visible. There are still highlights in a low key photo however and they help to define the shape of your subject, but as a proportion of the image space they are less significant than the shadows.
Images that are mainly light tones (with few, if any dark tones) is referred to as high key. Shadows are light and airy, often brighter than a mid-tone. (Shadows, as with highlights in low key images, help to define the shape of your subject so high key images can’t do without them entirely.)
The simplest way to shoot either a high or low key image is to apply suitable exposure correction (positive and negative compensation respectively) until the desired effect is achieved. However, this isn’t always a satisfactory solution. It’s fun to experiment this way but you’ll quickly notice that this technique has limitations. You’ll find, for instance, that the highlights may burn out long before the shadows are light enough to create a high key effect.
Ultimately, for complete control over the creation of high or low key image you need to look at how your subject (and its background) is being lit, modifying the lighting to suit.
To create a high key images you need light, lots of light, ideally supplied by two or more light sources, such as flashes or studio lights. The background needs to be lit, your subject needs to be lit. You may also need lighting pushed into the shadows. The er… key is to balance the relative brightness of your various lights to achieve the effect you want.
Changing the relative brightness of your lights means adjusting their power. With flash, this can be done by changing the power output. This is easier to achieve when using manual flash exposure, as you can adjust the power of each of your flashes independently until the light levels are right. Try using a lighting ratio of 1:1 initially, which is when the various flashes illuminate the subject equally, or 2:1, when the main flash is one-stop brighter than any of the other flashes.
Reflectors are useful tools for creating high key imagery as you can use them to ‘bounce’ light into the shadows to soften them.
Use studio lighting, either fluorescent or LED, and you need to dial in the right power settings until the most pleasing effect is achieved. This is generally far easier than with flash as you can see how the illumination falling on your subject changes as you make your adjustments. The one big drawback to studio lighting is that it is (generally) less bright than flash, which makes it slightly more difficult to overpower ambient light should this be necessary.
Whichever type of light you use though try to soften it as much as possible by using a softbox, shining the light through a diffuser, or even bouncing the light off a handy – preferably white – wall. (You do carry one of those around in your camera rucksack, don’t you?)
Creating a low key image is slightly trickier as you need to actively stop light from falling onto your subject (other than light necessary to define the shape of your subject). Flash or studio lighting can be modified by adding accessories such as snoots (essentially a long tube fitted to the front of a flash to create very directional lighting), or barn doors (a set of four flaps on the front of a light that can be opened or closed, again to create directional lighting). Ambient light can be reduced either by shooting with curtains or blinds covering windows or by shooting in the evening.
Shooting high or low imagery is slightly more challenging than ‘standard’ photography, but it’s fun and it creates atmospheric images. The trick is choosing subjects suitable for either approach. A slightly clichéd approach, but one which is effective, is to shoot women and children in high key and men in low key lighting. However, there’s a lot to be said for mixing things up a little bit and reversing this. Once you get the hang of one or both styles you may never want to shoot boringly average tonal range photos ever again.