Exposure

Exposure refers to the combination of shutter speed and aperture resulting in a certain amount of light reaching the sensing media of the camera.

Generally a good exposure is defined by an image that has a wide range of shadow and highlight detail within it.

A camera contains a light sensor that 'reads' the light coming from the scene it is pointing at and determines the correct aperture and shutter speed settings to achieve an even exposure. Most systems do this by breaking the image up into segments and reading the light coming from each segment. While this generally produces a good result, it may be tricked by a very bright area of the image such as the sun, or a very dark area of the image. This method is generally called multi-segment metering.

Most cameras also have at least one other option of metering an image, centre-weighted. With this system, the entire field of view is considered, but the central portion of the image has the majority of influence over the exposure. Generally this will be approximately 25% of the total image.

Some cameras also have the option of spot metering an image. This literally means using a small spot in the centre of the image (typically 5% of the total image) to meter from. Care should be taken when using this system, as it is very dependent on that tiny central portion of the image. Detail in the rest of the image may be completely lost as a result.

It is not always advantageous to have the 'correct' exposure for a given image. Most cameras have the ability to overexpose (make an image lighter) or underexpose (make an image darker) on purpose, over-riding the camera's chosen settings. This can be necessary to create a certain mood or effect within a picture, or to help the exposure system of a camera in difficult situations.

As an example, imagine taking a picture of a freshly snow covered scene. There is a lot of light being reflected from the snow and perhaps the sun is shining. The exposure system thinks that the overall scene has too much light, so allows less light to reach the lens. The picture you take results in snow with more of a dull grey appeance to it, than the crisp white you saw. This is because the exposure system has been fooled by the amount of light in the scene. It does not 'know' that the snow should be bright white, so makes adjustments to achieve an even exposure.

The way around this is to slightly over expose the image using exposure compensation controls. This is measured in eV and will generally change in 0.3 or 0.5 eV steps. The opposite of this might be when trying to take a picture of a figure in silhouette. The camera will naturally try to increase the exposure to bring some detail to the figure. Again, the camera does not 'know' that you want the figure to be a silhouette. To compensate, it is necessary to apply perhaps -0.3eV to darken the image and return the figure to shadow.

Most digital cameras have a convenient way to change the exposure applied to an image in one step, this is called exposure bracketing. This function will take three images in quick succession. One at the 'correct' exposure for the image and then one overexposed and one underexposed. The amount of over- and underexposing is generally set when the function is chosen and can be changed from a subtle variation to a strong one. Normally only a subtle change is required to get the desired result. While the camera is taking all three exposures, the shutter should be held down throughout and the camera kept as still as possible.

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