Exposure 1: Getting to grips with exposure

The basics of exposure are to do with light: how much there is in the image and how much is allowed to enter the camera via the lens. In automatic mode on a digital camera, the amount of available light is measured by the camera's in-built light meter. This information is then analyzed by the camera to determine the correct aperture and shutter speed for the scene. Aperture refers to the diaphragm within the lens that allows light to pass to the camera's image sensor. The aperture can be made narrower or wider depending on the requirements of the shot. The shutter speed refers to the blades behind the lens that open and close to allow the light to pass into the camera (although in the majority of digital cameras this is actually a switch that tells the image sensor how long it should be activated for). Shutter speeds are measured in seconds or fractions of a second.

Measuring the amount of light in a scene is the first step in ensuring a correctly exposed image. Within digital cameras there are normally a number of options for doing this (consult your camera's manual to see the full range for your camera). The first one is the matrix method, which in a lot of cases will be the default method used if the camera is on automatic mode (in which case you will probably not even be aware of it being used). The matrix method works by looking at areas within the whole scene and calculating the amount of light accordingly. This is done by comparing the image with a database compiled from thousands of images taken in a vast range of lighting conditions. The matrix method compares the current scene with those in the database and when it finds a similarly lit scene it applies the required shutter speed and aperture settings. The matrix method can be used successfully for the majority of photographic situations but it is not foolproof and can be confused by complex lighting conditions.

Another method of measuring light in a scene is center-weighted. This also looks at the whole image but gives extra prominence to the center of the scene. This can be useful if you want to make sure the center of the scene is correctly exposed while still retaining an acceptable exposure for the rest of the image.

The final method of metering is spot-metering. This works by measuring the amount of light in a very small area within the center of the image. This can be used if you have a very light subject against a dark background (or vice versa) and it is essential to get the main subject correctly exposed. It may result in the background being either under or over exposed but that is the trade-off for this type of metering.

For a lot of compact digital cameras you may not even be aware that your camera is taking a light reading, but this is what it does when you focus an image in automatic mode. To start taking a bit more control over your light metering and exposure settings, try using the different metering options on your camera to see how these effect the final images.

Once you are aware of how your camera operates in different metering modes you can then move on to taking light readings from specific areas within an image. For instance, if you have an image of great contrast with some very dark and some very light areas, it may be best to take a light reading from an area between the two. This is known as a midtone. However, if the midtone area is not at the center of the image then this could cause problems, particularly for spot or center-weighted modes. To solve this it is necessary to use the exposure-lock technique. This can be done manually by pointing the camera at the area you want to take the light reading from and half depressing the shutter release button. Then, with the button still held halfway down, recompose the scene to the composition you want and capture the image (this also has the effect of locking the focus too and can be used to focus off-center subjects). Some cameras also have an Exposure-Lock (EL) button that can be pressed once you have pointed the camera at the area from where you want to take the light reading. This locks the exposure at this point and you can then recompose the scene to capture the image you want.

Once a light reading has been taken the camera can then automatically set the aperture and shutter speed to produce a correctly exposed image. However, both of these functions can be set manually, for specific photographic effects. This is discussed further in Exposure 2: Depth of Field.