Photography Tech Tip : Depth of field

This is one in a series of 'Tech Tip' articles intended as a guide to digital camera technology and terminology. Its aim is to demystify some of the language used on this site to allow you to make more informed decisions when considering purchasing a new digital camera.

DEPTH OF FIELD
The term 'depth of field' refers to the range of an image that is acceptably in focus.

The image above depicts a broad or wide depth of field. The zone of acceptable focus spans a broad range of distances from the camera lens. In reality, perfect focus is achieved at a specific distance from the lens, but any object within the zone is sufficiently in focus to be considered acceptable. A broad depth of field is achieved (or caused) by reducing the aperture (see Tech Tip: Aperture).

This second image depicts a narrow or shallow depth of field. The image contents are the same and each object is the same distance from the camera, but only the centre circle is in focus. This is achieved (or caused) by increasing the lens aperture.

The effect of depth of field is also relative to the distance from the lens. For very large apertures, it may be possible to reduce the depth of field to just a few millimetres for subjects that are relatively close to the lens.

A practical example of depth of field is shown below. Both pictures contain the same objects and are taken from the same distance. The only difference is in the aperture settings, causing different depths of field.

        

The image on the left is focussed on the foremost flower. One's view is automatically drawn to this as it's the only area of the image in crisp focus. Compare this with the image on the right, where the plant in the background (and even the wall) is in focus. The difference is most noticeable in the pattern on the furthest plant pot.

In portraiture a narrow depth of field is often preferable to draw attention to the subject by blurring unwanted areas of the image (background and foreground), thus removing possible distractions from view. For landscape photography, a broad depth of field is often required so that areas of varying distances from the camera remain in focus.