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Understanding Exposure

The dictionary defines exposure as “an act or instance of being uncovered or unprotected.” In photographic terms the thing that is being uncovered is the film or, with modern DSLR, the sensor. What the film or sensor is being exposed to is light.

If the sensor is exposed to too much light then the resultant image will be bleached or washed-out (over-exposed) and, conversely, if it is exposed to too little light the results will be dark or black (under-exposed). Exposure, then, is not about some fixed standard, but rather the skill of using the camera to create the exposure that you feel best suits the photograph you are taking.

Over and under-exposure can be used to your advantage if you understand them and apply them for artistic ends. Therefore, in order to attain a correctly exposed picture we need to control the amount of light being allowed in to the camera and on to the sensor.

The amount of light permitted to the sensor is controlled by three key factors:

Shutter Speed Aperture Size (f-stop) ISO

The first is shutter speed which refers to how long the shutter is left open, allowing light to pass to the sensor.  The second factor is the F-Stop or aperture, which refers to the size of the hole through which light can pass. Finally there is ISO which determines the sensitivity of the sensor to light. It is by balancing these three measurements correctly that we can attain a properly exposed photograph.

Shutter speed and  aperture size affect the photograph in two distinct ways.  For example, if you take a photograph of a fast moving object then you will need a fast shutter speed. This means that the shutter only opens for a short length of time and you can freeze the action without  blurring  the image. 

However, if you wanted a photograph of the very same moving object to create a blurred effect then you would need a slower shutter speed so that some of the movement is registered by the sensor during the time the shutter is open. 

Shutter speeds range anything between 1/4000th  of a second (for some wildlife and sports photography) right up to 30 or 40 seconds, although there are plenty of examples where shutter speeds can last minutes or even hours. For, example, the silky texture you will often see  on moving water is usually captured by opening the shutter for between 3 to 10 seconds.       


Aperture Size is a particularly important part of the calculation in landscape photography because the smaller the aperture, the greater the plane of focus, or Depth of Field.  This is how much of the image you want to fall in focus. However, by decreasing the size of the aperture the shutter is required to stay open longer to expose the sensor to the correct amount of light.