Camera College: Depth of field
Accurate focusing and fast shutter speeds can help to improve the sharpness of your pictures. But choosing the right depth of field is also crucial…
How to control the focus in your shots
Although a camera lens can only be focused at a single distance each time you take a picture, sharpness doesn’t start and stop at that point. Instead, it extends both behind the object in focus and in front of it, towards the camera, until things start to look blurred. It’s the extent of this ‘apparent sharpness’ that’s known as the depth of field – or the DoF.
The amount of depth of field varies from picture to picture, depending on a number of different factors. Sometimes it can be shallow (where only a sliver of the picture appears sharp), sometimes it can be extensive. Being able to control which parts of your picture are sharp and which are blurred allows you to draw attention to features that you want a viewer to look at, and to hide elements that would otherwise be distracting. You might want to capture the maximum depth of field possible when you photograph a landscape, for example, in order to record crisp details from the foreground through to the horizon. But you’ll probably want a much narrower band of sharpness when you shoot someone’s portrait, to help separate them from the background.
While there isn’t a depth of field control on your camera, there are a number of camera settings that determine the extent of the sharpness. These include the size of the
lens’s aperture, how close or how far the lens is focused, and even the type of camera that you’re using.
Changing the aperture is usually the most straightforward and convenient way to control the depth of field, as you don’t have to change your camera setup or your shooting position. The best way to set the aperture is to use the camera in either Aperture Priority or Manual mode, although you’ll usually need to use your camera’s depth of field preview facility to see the effect any change has on the image.
You can reduce the zone of sharpness and capture a shallow depth of field by using a large aperture (such as f/2.8 or f/4). If you want to make sure you’re capturing a greater extent of sharpness, close down the aperture to a smaller setting (such as f/16 or f/22) to increase the depth of field. The lens you’re using will dictate which apertures are available.
The distance at which a lens is focused also has a pronounced effect on the depth of field. The closer you focus, the narrower the band of sharpness becomes, to the point where the depth of field may only cover a few millimetres when you’re shooting tiny subjects with a macro lens. The effect is more pronounced if the background is far away from the subject.
Focal length also has an impact on the depth of field, although it depends on where you stand to take the picture. For instance, a 200mm lens focused on an object 10 feet away will have a wafer-thin depth of
Changing the aperture is usually the most straightforward and convenient way to control the depth of field, as you don’t have to change your camera setup or your shooting position
field compared with a 20mm lens focused at the same distance. But if you move closer with the wide lens and farther away with the long lens so that object occupies the same proportion of the frame, the depth of field will be roughly the same. The image taken with the long lens will still appear to have a shallower depth of field due to the narrow angle of the lens – it will fill the frame with a much smaller area of background, so any out-of-focus are magnified as a result.
Another feature that has an effect on the depth of field, but which is largely out of your control, is the size of the camera’s imaging sensor. Depth of field appears shallower with larger sensors, as you need to be physically closer or use a longer focal length to achieve the same image size as you’d get using a smaller sensor. This is why a full-frame camera gives a much shallower depth of field than an APS-C or Micro Four Thirds camera that is set to an equivalent focal lengths and aperture. This means that cameras with smaller imaging sensors have an advantage when it comes to making more of a scene appear sharply focused.
Of course, you can combine all of these settings and attributes to create dramatic shifts in the depth of field in your picture. To capture a very narrow band of sharpness and make backgrounds appear artistically blurred, use the longest focal length you can get away with, set the largest aperture possible on the lens, get close to your subject and frame them against a distant background. To maximise the depth of field, combine a short focal length with a small aperture, and don’t focus on anything too close to the camera. Experiment and you’ll soon get the hang of it.
A full-frame camera gives a much shallower depth of field than an APS-C or Micro Four Thirds camera at equivalent focal lengths and apertures