The Exposure Triangle
Exposure is the technical term for the creation of a photographic image. An exposure can be deemed underexposed, correctly exposed or overexposed although, as photography is a creative art, this is a subjective judgement.
Therefore, a correctly exposed photograph is one that is as the photographer intended. An underexposed image has darker tones than it ‘should’ have, while an overexposed one is too light in tone.
An exposure is created through the combined effects of the aperture, shutter speed and ISO settings. These interact to create a brighter or darker image, in a relationship often called the ‘exposure triangle’.
Your task as the photographer is to control these three elements so that the correct amount of light contributes towards each exposure you make, in the way that you intended.
In a scene where there is less available light, you can manipulate the camera settings to maximise the amount of light contributing to the exposure, so that the image isn’t underexposed. Conversely, on a very bright day, you can adjust these settings to minimise the amount of light contributing towards the exposure so it doesn’t become overexposed. In both cases, the final image should look as planned – often the photographer’s intention is to capture an image that reflects how the scene appeared to the human eye.
Whereas our eyes automatically adjust to changing light levels, cameras use algorithms to judge the exposure settings requires in order to render an average mid-tone correctly in the ambient light. While the camera can often make a very good job of this, it will struggle in certain situations, such as when there is bright light behind your subject or when a scene has a high level of contrast.
Each of the three key elements that contribute to an exposure – aperture, shutter speed and ISO – has ‘side effects’. Changing the aperture affects the amount of the image that is in focus, while changing the shutter speed impacts on how motion is recorded, whereas changing the ISO impacts on the image quality. Allowing the camera to make the exposure decision for you means you won’t be in control of any unwanted side effects caused, and you will also miss out on the ability to use them to your advantage.
Histograms can help
A histogram is a graph which you can display over or next to your images, either on the back of your camera or in photo editing software. The graph shows the spread of pixels across the different tones. An image with a lot of light tones will have more pixels towards the right hand side, whereas a darker toned image will have more on the left. An image with a wide range of tones will have a spread of pixels across the graph. There isn’t a correct shape to look out for; rather, you can use the histogram to help you judge whether the spread of pixels looks right for that particular scene.
If there are pixels stacked at either edge, this warns you that you are likely to have lost image detail in this area. Lighter-toned areas of the image may appear pure white and/or darker-toned areas may appear pure black. This occurs when the exposure settings aren’t suited to the amount of light available, or when the scene has a high level of contrast. Many cameras now show blown out highlights as flashing areas on the image preview when you review the image.
Cameras have a limited ‘dynamic range’, which means they can’t process the same extremes of light and dark tones in a scene as a human eye can. In a backlit or high contrast scene, the camera will only be able to accurately capture a limited portion of the visible detail. This means some areas of the image will be recorded as just pure white or black, with none of the subtle variances in tone or detail. In this situation, the histogram will show stacked pixels on both the left and right hand edges. You have two options – take two different exposures to capture each end of the tonal scale accurately and combine them in photo editing software, or accept that you will have to compromise on image detail at one end of the scale at least.