Digital cameras control exposure using both shutter speed and aperture. Why both?
Wouldn’t one or the other do the job? There are creative advantages to these two means of exposure control. Smaller lens apertures offer more depth of fi eld (near-to-far sharpness), while fast shutter speeds let you freeze fast-moving objects. Shutter speed and aperture are interchangeable, so that if you want to use a smaller lens aperture, you can compensate with a longer exposure. Or, if you want a shorter exposure, you simply set a wider lens aperture. For example, if your camera indicates an exposure of 1/250sec at f/8 but you want to shoot at 1/1000sec, which is two stops, or EV values, faster, you need to increase the aperture value by two stops as well, to f/4. Some cameras allow you to adjust shutter speed and aperture values in 0.3 EV steps, but the same principle applies – a change in one must be mirrored with a same-sized change in the other.Adjusting exposure
So how precise do you have to be with exposure? Even though digital cameras only have a certain amount of ‘exposure latitude’, in practice there are many different ways of interpreting a scene, and many exposure errors can be rectifi ed or at least improved with a bit of image-editing. To give you an idea of how the subject brightness changes with exposure, here’s the same scene at seven different exposure values, all shot at the same lens aperture, but with shutter speeds 0.5 EV apart. These also demonstrate the idea of exposure latitude and dynamic range. There isn’t one shot where detail’s been recorded both in the foreground and the garden outside – the scene is outside the dynamic range of the camera’s sensor. You might prefer the ‘overexposed’ shot because it shows the subject’s face with a nice high-key effect, or a darker silhouetted version. Or you might open one of the in-between shots in Photoshop and attempt to balance the tones more evenly.
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Source Digital Camera Magazine - Master Exposure
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