Creative depth of field
To take control of your image sharpness, you need to get to grips with depth of field. Tim Daly shows how it’s an easy way of improving your photographs
Determined by your lens aperture setting and your proximity to the subject itself, depth of field is unique to photography. When looking around us, the human eye focuses so quickly on subjects at different distances that we hardly notice any transition. In fact, we are blissfully unaware that we can’t keep sharp focus on a close-up object and the distant background simultaneously.
With photography, however, you can create an image that presents more information to the human eye than happens in reality.
Depth of field is a photographic term used to describe the range in which two objects at different distances look equally sharp, and can be varied from a matter of millimetres to infinity. To use this technique, you need to control the aperture.
Aperture and depth of field
The aperture is a variable-sized circular opening inside your camera lens used to moderate existing light levels for a successful exposure, and it also determines depth of field. A typical lens has an aperture range like f2.8, f4, f5.6, f8, f11 and f16.
In terms of controlling exposure, an aperture of f2.8 is the widest and lets in the most amount of light. At the f16 end of the scale, the aperture is at its narrowest and lets in the least amount of light. In depth of field terms, f2.8 produces the shallowest result with little sharp detail beyond your chosen subject. At the f16 setting, sharper detail is rendered both in front of, and beyond, your main subject.
On budget digital compacts there’s usually a reduced set of apertures to choose from, such as f4 and f11. On better compacts this will extend to five or six options, with a full range available on the top-price SLRs. Aperture values are usually accessed via a thumbwheel or menu on the rear of the camera body, but can only be selected in manual or aperture priority exposure modes. In auto and other programme modes, the camera meter decides on an appropriate aperture value to generate a correct exposure, without taking your depth of field wishes into account. The aperture priority exposure mode lets you take control of depth of field and leaves you to select a shutter speed.
The other factor in creating depth of field effects is your distance from the main subject. If a landscape image is divided up into foreground, middleground and background, then it’s important to realise that you can’t separate objects lying in the same plane with shallow depth of field effects. As subjects get further into the background, it becomes harder to assign sharpness to one element and not the other. For mid-range zoom lenses found on most digital compacts, anything more than five metres away will record with a similar level of sharpness as the furthest parts of the scene.
Focus points
Especially important when focussing at close range is a third factor: your exact point of focus. A common mistake when shooting portraits is to focus on the nearest part of your sitter, usually the tip of the nose, which can leave the eyes slightly unsharp. Instead, pick the nearest and furthest parts you want sharp, then focus one third of the way in.
Depth of field doesn’t remain constant throughout the far-reaching landscape and close-up photography. As you get closer and closer to your subject, the effective depth of field diminishes until it can be reduced to a matter of millimetres – even at maximum f16. If you want to capture detailed images at close range your small apertures will force you to use slower shutter speeds than normal, and necessitate the use of a tripod.
