During the most fortuitous shoots, a soft and even light illuminates your scene with an evenly gradated array of contrasts ranging from subtle highlights to softly spreading warm shadows. Such conditions are great to work with, testing neither your camera equipment nor indeed your eye. Spot metering such a scene at various points will render reasonably similar values throughout, meaning you can be sure that a healthy image is easily within the grasp of a single exposure.
Unfortunately, things are not always that straightforward. On a bright day our own vision struggles to read the brightest tones, and inky shadows hide impenetrable details revealed only through scrutiny. Your camera is vastly inferior at making sense of things; hence, a ‘one-shot’ approach will invariably lead to disappointment. Think of the simplest composition with a bright summer sky and a dark foreground – expose correctly for the sky and the foreground will be too dark; replicate for the foreground and the sky will be burnt out. These, and a whole host of more complex scenarios, are where exposure bracketing comes to the rescue.
Digital cameras often have an auto-bracketing setting, allowing you to open the shutter and capture a solo image which your camera will automatically create another two from – one slightly underexposed (often around -1/3 stop), and one slightly overexposed (often around +1/3 stop). Some cameras will allow a higher number of bracketed files from a single image and offer you the creativity to select your own exposure values. You could, of course, bracket manually by simply shooting a range of varying exposures, but the time taken to re-shoot could create problems with moving elements and/or changing light conditions.
Once you have your bracketed files, check (on your computer – not your camera!) that the underexposed files have correctly allowed for the brighter tones and vice versa concerning the overexposed ones with the darker areas of the scene. Using your processing software, the three (or more) files can be imported as separate layers, allowing you to mask off and select the areas you want from each with a view to combining the layers to one correctly exposed image across the entire scene.
The trick is to keep things natural, but allow insight, remembering that a photograph under challenging conditions should look just that. Don’t shy away from contrasts – just nurse them to a level where sense can be made.