18 October 2007 15:45
Differential focus is keeping a small area in focus to allow you to blur the background (or foreground), so that the main subject is more obvious in the picture. This technique is extremely useful when you’re out in the field.
You can try to frame your shot to minimise the distractions behind the subject, but you’ll often have to cope with what nature throws at you. This is where being able to control the amount of the image that’s in focus makes all the difference.
The amount of the picture that’s in focus is called the depth-of-field, and is determined by a combination of aperture, focal length and your distance from the subject. Selecting a wide aperture such as f/2.8 or f/4 gives the smallest depth-of-field, rather than the more normal f/8 or f/11 that you’d use for most standard shots.
Using a long focal length lens and getting closer to your subject will also produce a smaller depth-of-field.
Using all these elements together will result in only a small area of your picture being in focus. This will blur the background and help make your subject stand out, but your focusing technique needs to be spot-on.
Only a tiny amount of the picture will be pin-sharp, so ensure that you focus on the most important part of the subject. For example, use the petals of a flower or the eyes in a portrait to focus on and you’ll get the most out of this technique.