by Steve Franks

Created on: July 23, 2008

One of the most difficult concepts for beginners to understand is depth-of-field. The human eye compensates for many different things and depth-of-field is one for the most important.
When you look at anything your eye sends multiple images to the brain in quick succession, so that everything appears sharply focussed. In fact our eyes only see through a small part of the retina called the fovea. The lens continually scans and focuses what it is looking at onto this area effectively combining a large number of images into a single view.

Cameras do not have this facility, their lens focuses the subject onto the sensor according to the laws of physics. The amount of detail that can be shown sharply is determined by the lens aperture, focal length and the size of the camera's sensor.
A larger aperture (smaller f number) has less depth of field than a smaller one. i.e. Depth of field is less at f2.8 than at f8, which is less than at f32.
When you use the automatic, or program, modes on your camera it will make a number of compromises. These are designed to ensure that you obtain some sort of picture under any conditions. Usually it will set a shutter speed high enough to stop camera shake, then work out if it can stop the lens down to a smaller aperture. This is ideal for taking pictures of sports, children playing, animals etc. but won't always give pleasing results for landscapes, still life and other static subjects.
Aperture and shutter speed are inversely linked. For any given level of light one will go down as the other rises. If f2.8 gives correct exposure at 1/1000 second, so will f4 at 1/500 and f8 at 1/250. If your camera has a display that shows the aperture and shutter speed experiment with different settings and compare the results. You will find that the different setting give images that look the same at first glance, but show different details when you zoom into them
Many cameras have scene modes that include a landscape setting, which will maximize depth of field. That's OK for landscapes, but one doesn't always want maximum depth of field. A shallow depth of field (low f number) allows you to blur the background, so that it does not distract from your main subject. This is especially useful if you zoom in, because telephoto lenses have less depth of field, than wide angle.
Macro lenses have least depth of field of all, I have one on my digital SLR that only has 1mm (about 1/25th inch) wide open at its closest focusing distance. This means that even small bugs are not completely in focus and aperture priority is a must, stopping it down to f8, or f16, makes a little bit more of the image sharp. This can make the difference between a photograph that is deleted, because it looks blurred and one that is acceptable as the important details are in focus.
Aperture priority is not magic, it is not appropriate for every photograph you take, but I have found that I leave my camera in aperture priority mode most of the time. I can honestly say that if my camera had no other modes I would still be able to do everything I want to with it. Reducing the f number will provide a high shutter speed to stop camera blur, freeze subject movement, or make the subject stand out from the background. Stopping the lens down to a smaller aperture will give me great depth of field for landscapes and macro photography.

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