Exposure Explained

What is exposure?
To make a photographic image a camera allows a pre-defined amount of light onto a light sensitive surface, either film or a digital sensor, which is then recorded as a picture. This is known as an exposure.

The exposure is critical to successful pictures; too much exposure will give you washed out or over exposed images. Too little will result in dark images, lacking in detail, (underexposed). Modern cameras are very good at working out the correct exposure for you and image modes or pre-set programs allow you to capture most scenes with a good chance of success, however a thorough understanding of how the camera achieves this will allow you greater control and creative flexibility.


Elements of exposure
There are two main elements to an exposure, both of which control how much light is allowed onto the sensor at any one time, although they both affect the final picture in different ways.

The two are directly related and in order to maintain a correct exposure, you cannot adjust one without adjusting the other. Depending on the image or effect you are trying to achieve it is necessary to balance out the two.

The first element is shutter speed. This is the amount of time that the sensor is exposed to the light and is measured in fractions of a second. A typical camera will have a range from maybe 1 or 2 whole seconds down to 1/2000 of a second. More expensive models usually feature faster speeds.

The longer the shutter speed, the more light is allowed onto the sensor but this can also have an effect on the image itself. A fast shutter speed will usually result in a sharper image, but even in 1/1000 of a second an object can move and result in a blurred image, once you get below about 1/125th of second the camera can also record the movement of your hand, causing the same effect.

This is known as camera shake. Longer exposures are useful in low light conditions but can render moving objects blurred. This can be used to great effect however to show movement.


The second element of exposure is aperture. The aperture is a diaphragm within the lens that opens and closes to allow more or less light in as required, rather like the pupil of an eye. Apertures are measured in f-stops, for example f4, f8, f16 and so on. Your lens will probably show an f number on the front, which is its widest or fastest aperture.

Again, the aperture controls how much light is allowed onto the sensor whilst the shutter is open, but it also has another crucial role to play. As the aperture is a physical part of the lens, it has a direct influence on the way the lens focuses an image. This is known as depth of field (DoF). The DoF is the area (in distance) of the picture which is in focus. Smaller apertures result in less, or shallow depth of field but let in more light. Larger apertures result in greater DoF but let in less light.

A further element to consider is the ISO speed. ISO speeds go back to the days of film photography and were a measure of how sensitive a particular type of film was to light. An ISO 400 film is twice as sensitive as an ISO 200 film for example. The trade off here is that in order to make the film more sensitive, the physical size of the grains that make up the film had to be increased, resulting in a noticeable grainy quality to the image.

The same is still true with digital photography, although there is no film grain to consider, increasing the ISO increases the cameras sensitivity to light. As you increase the ISO rating, you are able to get the same exposure with a faster shutter speed or larger aperture, but the quality or detail of the image decreases. With digital cameras rather than increased grain you get increased ‘noise’, although the two appear similar.