Photography Tech Tip : Exposure
This is one in a series of 'Tech Tip' articles intended as a guide to digital camera technology and terminology. Its aim is to demystify some of the language used on this site to allow you to make more informed decisions when considering purchasing a new digital camera.
EXPOSURE
In digital photography, exposure is the relationship between the amount of light and the duration of time that the sensor has to form an image. There is an optimum exposure level; too much time or light and an image will be 'overexposed'; too little of either and it will be 'underexposed'.
The three main components of exposure in photography are lens aperture (see Tech Tip: Aperture), ISO level (see Tech Tip: ISO) and shutter speed. Increasing aperture allows more light to enter and less time is required (faster shutter speed) to form the image, and vice versa. Adjusting the ISO level (sensitivity) of the sensor also affects the amount of light required to form the image, allowing faster shutter speeds, or smaller apertures, in low-light conditions, but at the risk of introducing noise. For this reason, maintaining an ISO level of 100 or 200 is recommended.
Correct exposure is the optimum balance between shutter speed and aperture at the lowest possible ISO level that allows all the tones and colours within a scene to be recorded most accurately. Varying one component requires an equal and opposite variation of the other to maintain correct exposure. E.g. Reducing the aperture from f/1.4 to f/2 - one f-stop (see Tech Tip: Aperture) - reduces the amount of light by half and requires halving the shutter speed (doubling the time), perhaps from 1/250th to 1/125th of a second. The actual values will vary on the conditions. Changing one without the other can result in over- or under-exposure.
However, slight adjustments to exposure can create more artistic results or are required in certain situations. Most cameras have exposure compensation settings, allowing you to manually adjust exposure away from the optimum, for this purpose. In practice, slight under-exposure can deepen the colours in an image; over-exposure may be necessary when the subject is backlit (bright background). In climates such as Australia, where sunlight can be very bright, many photographers set their cameras to slightly under-expose permanently to compensate.
Exposure compensation controls use exposure value (EV) as a measure of under- or over-exposure, with 0 indicating no compensation is used. Most compact digital cameras allow settings between + (overexpose) and - (underexose) 2 EV, and some digital SLRs extend the range to +/- 5EV. Adjustments are in 1/3 or 1/2 EV steps. E.g. -2, -1.7, -1.3, -1, -0.7, -0.3, 0, 0.3, 0.7, 1, 1.3, 1.7, 2.
EXAMPLES
The following 3 shots show exposure compensation in action from -3EV to +3EV respectively; the middle shot is without any compensation.
