Master depth of field

Matthew Richards shows how controlling depth of field opens up a whole new world of creative opportunities

Control what’s in focus in your images

In a nutshell, the depth of field is a measure of the physical distance between the closest and furthest points in a scene that will be rendered sharply in an image.

With a larger depth of field, more of the scene will look sharp from front to back. The size of the depth of field is governed by three things: The first is the focal length or zoom setting of the lens, the second is the focus distance, and the third is the aperture setting.

A lens or zoom setting with a shorter focal length will result in a larger depth of field. Wide-angle lenses therefore tend to give a larger depth of field – telephoto lenses have the opposite effect. Narrower aperture settings stretch the depth of field, whereas wide apertures shrink the depth of field. And finally, the depth of field also shrinks at closer focus distance settings. For example, ‘macro’ lenses deliver a very small depth of field when shooting close-ups.

Despite all this, a regular prime or zoom lens can only render objects with optimum sharpness if they’re at a position that equates to the exact focus distance. Depth of field is a measure of distances at which objects appear to be ‘acceptably’ sharp within images.

The formula for what is an acceptable degree of sharpness is known as the ‘Circle of Confusion’. A pinprick of light in an image would be rendered as such if it was sharp in an image. If it’s out of focus, it would appear as a circle, lens aberrations aside. The more the pinprick of light is defocused, the larger the circle would be. The Circle of Confusion equates to the diameter of this circle. If it’s small, the object is acceptably sharp.