Understanding Depth of Field in Photography

Put simply, the ‘field’ in ‘depth of field’ is the area of a photo that is acceptably sharp. This is actually a deceptively complicated area of photography, but here we’ll try to explain it as simply and clearly as possible.

When a camera focuses on something, it is ensuring that light coming from that something converges in such a way that it will look sharp in the ensuing photo. In fact, it’s not only that one thing that will be in focus, but everything that is in the curved plane that runs through that thing and is an equal distance from the camera lens. Depending on a couple of things, this plane might be shallow (e.g. everything from a distance of 10 inches from the lens to a distance of 11 inches from the lens is sharp; everything else is blurred), or it might be deep (e.g. everything from three feet from the lens all the way to the horizon is sharp; only things that are closer than three feet are blurred).

How shallow or deep depth of field is for a given photo is determined by focal length, aperture, and focusing point. So with a telephoto-type focal length (this is usually regarded as anything beyond 50mm), a large aperture setting (for example, f/2.8), and a focal point close to the camera, depth of field will be very shallow. Again, this means that the point of focus and anything a similar distance from the lens will be sharp, but anything significantly behind or in front of that plane will be blurred, and the amount to which those thing are blurred will increase the further they are from the plane of best sharpness. The opposite settings will give the opposite effect: a wide-angle focal length at a large aperture (say f/11) will have a massive depth of field – everything is likely to be acceptably sharp.

For most photography, the desire is to have everything in the scene sharp. This means selecting a small enough aperture so that the depth of field encapsulates everything in the scene. Sometimes however, narrow depth of field is desirable, for example in portrait photography, where the intended effect is for the subject to be sharp but the background rendered as an out-of-focus blur.