Depth of field and where to focus
Choose a depth of field to suit where you want your images to be sharp.
Depth of field refers to the extent of the sharpness of an image. If an image is noticeably soft in front of and behind the point of focus, then its depth of field is shallow; if the image is sharp from front to back, then its depth of field is broad. Depth of field decreases the closer you are to a subject, so accurate focusing is vital to ensure that your subjects are sharp. This is why narrow apertures are used in macro photography (see below).
Even then, when you shoot extreme close-ups, the depth of field may be only a matter of millimetres. As a rule of thumb, there’s roughly double the amount of depth of field behind a focal point than there is in front of it. See what happens if you were to shoot a landscape (right) with a narrow aperture of f/16…
1 LARGE APERTURES:
F/1.2 TO F/4
If you want the areas in front and behind your subject to be out of focus – where the effect of ‘bokeh’ is pronounced – then you need to select a wide aperture, such as f/2.8 or f/4. Use this aperture range for portraits, or for any situation where you want to separate your subject from its background – like macro or wildlife photography.
2 MEDIUM APERTURES:
F/8 TO F/11
Mid-range apertures like f/8 tend to give you the best optical performance, and will be close to the ‘sweet spot’ of a lens (see panel, right). Choose an aperture in this range and you’ll get a medium depth of field and the ability to use shutter speeds fast enough to give you sharp images.
3 SMALL APERTURES:
F/16 UPWARDS
To maximise the depth of field and get as much as possible of the scene sharp, you need to select a narrow aperture, ideally f/16 upwards. Apertures of f/16 also give a pleasing ‘star’ effect when photographing lights at night; but be aware that due to an optical effect called diffraction, smallest apertures won’t give the sharpest results.