Exposure Header

How to control exposure

Exposure is a measurement of the amount of light hitting your camera’s sensor and determines how light or dark your picture looks. It can be controlled by shutter speed, aperture and ISO. Making adjustments to any of these not only affects your exposure but also the look of your photo. 

This video explains how to use the aperture, shutter speed and ISO settings to get the right exposure and effect for your photo.

Your camera can usually solve all your exposure needs using Auto modes. However, learning how the different elements affect your photos will give you the confidence to take manual control. The resulting images will then stand out from the rest.

Aperture is the size of the hole in the lens through which light passes. Each lens has a range of aperture settings from large to small. Use Aperture Priority mode (Av) to control the aperture whilst still letting your camera look after overall exposure.

Aperture is measured as an f-number. Large apertures are expressed as small numbers, such as f/2.8, letting in more light and producing a shallow depth of field (increased blur beyond your point of focus). Small apertures are described by larger numbers, such as f/16, letting in less light and producing large areas in focus from foreground to background.

Aperture comparison0.4 sec, f/16, ISO 100
Aperture comparison1/250s, f/1.8, ISO 100

These two shots are exposed similarly but with different apertures. See how using a small aperture for the image on the left creates a greater depth of field with lots of in-focus areas. Enlarging the aperture in the image on the right reduces the depth of field, so much less is in focus.

Shutter speed is the length of time that light is allowed to fall on the sensor. It is expressed in seconds. Use Shutter Priority mode (Tv) to control the shutter speed whilst still letting your camera look after overall exposure.

Fast shutter speeds such as 1/1000s are used for freezing fast action such as flowing water. Slow shutter speeds are used for adding movement such as blurring flowing water in a stream.

Shutter speed comparison1/1000s, f/5, ISO 2000
Shutter speed comparison1/2s, f/16, ISO 50

Compare these two shots taken with very similar exposures but with different shutter speeds, see the effect of using a slow shutter speed on the flowing water. For long exposures you will need to use a tripod or support to prevent camera shake.

ISO sets the light sensitivity of the sensor. It can therefore be used to amplify the light hitting the sensor allowing greater flexibility with shutter speed and aperture. The ISO typically starts at a figure of 100 and rises as the sensitivity increases.

ISO can also deteriorate the image quality in the form of grain, or noise, in your images and muted colours. It’s generally best to use the lower ISO settings if possible.

ISO comparison1/20s, f/5.6, ISO 400
ISO comparison1/20s, f/5.6, ISO 1250

The scene on the right is a lot darker than the one on the left. To maintain a shutter speed fast enough to shoot without a tripod you will need to increase the ISO to compensate. This time to 1250, which hasn’t affected the picture quality unduly.

Controlling exposure is a juggling act between these three settings. For example, increasing the shutter speed to freeze fast action may underexpose your image. To compensate you need to increase the aperture. Similarly, decreasing your aperture to maximise front-to-back focus will require a reduction of the shutter speed. You can also use the ISO to maintain exposure. You will often see this relationship illustrated as the exposure triangle.

The Exposure Compensation dial enables you to lighten or darken your picture. A minus figure will decrease the exposure and darken the image, a positive number increases the exposure to lighten it.

Exposure comparison1/15s, f/5.6, ISO 400 Exposure -2
Exposure comparison1s, f/5.6, ISO 400 Exposure +2

These two shots have clearly different exposures despite the same amount of light in the scene. The shot on the left is too dark or underexposed and the shot on the right is too light or overexposed. Using exposure compensation would correct the exposure.

Adjusting exposure in this way can be used for creative effect. Darken your images to increase mood and deepen colours especially in landscape photography. Increasing your exposure will brighten the shadows and produce a more modern feel.

Exposure bracketing enables you to capture a scene at a number of exposures and choose which one works best afterwards. Choose between 1 and 3 stops on the exposure scale and allow the camera to make the necessary adjustments to your settings.

Next time you take your camera out, try experimenting with the different settings and see for yourself the changes it can make to your photos. Don’t forget once you have taken some shots you are proud of, share them by uploading to the Canon Gallery.

Please note: In some cases the values shown for the images are representative to illustrate the different shots taken