When photographers talk about ‘depth of field’ (DoF) they are actually referring to the ‘range of distances from the camera which APPEAR in sharp focus’. (In reality, there is no such thing as DoF, as a lens can only focus on a single point at any one time. ) So, for the photographer, what affects DoF?
1) Aperture. Altering the size of the hole in a camera lens through which the light passes is the main way in which photographers control DoF. Larger apertures permit light to enter from a wider range of angles. This means that objects in front of or behind the focal point will fall out of focus more quickly, hence DoF is reduced. Conversely, reducing aperture limits the angles through which light can pass, increasing DoF.
2) Focal length. It’s worth remembering that the focal length of your lens will also affect DoF. A longer (telephoto) lens will not only bring objects closer in a photograph, but will also bring objects at different distances closer together. This means that out of focus objects will be magnified, hence reducing DoF. The opposite also applies. Short focal length – wide angle – lenses tend to have much greater DoF at any given aperture.
3) Focus distance. If you focus your camera to a point very close by, light must be brought to focus much more sharply than when focussing on something far away. This accentuates any differences in focus, and means that the closer you focus, the more DoF is reduced. Macro photographers who attempt to focus on tiny objects just a few centimetres from the lens have particular issues with DoF which can be less than a millimetre!
How do these three factors interrelate, and how can you estimate DoF for yourself? The relationships are complex, and I suggest you experiment with a DoF calculator (many are available online) which will enable you to try out different combinations of aperture, focal length and distance, and see how they affect DoF. Some SLR cameras also have a DoF preview function, which simply stops down the aperture to the preset value so you can see the intended DoF for yourself.
How do I maximise DoF? If you want to take a nice landscape, and ensure everything is sharp from foreground to background, then you need to know about ‘hyperfocal’ distance. By definition, the hyperfocal distance of a lens (for any given combination of focal length and aperture) is the distance from the camera which, if focussed on, permits maximum DoF. In fact, if you focus on something which is the hyperfocal distance away from you, then everything from HALF that distance to infinity will appear in sharp focus.
How do I work out hyperfocal distance? Again DoF calculators will tell you, but you won’t always have your PC with you on a photo shoot! In this case you might want to print out a simple table of hyperfocal distances from the web, which give these distances for various aperture/focal length combinations. Otherwise a great rule of thumb is to simply focus on something about 1/3 into the view you wish to photograph. If there is sufficient light to permit an aperture of F8 or above, you are unlikely to go wrong.
How do I take photographs where the background is deliberately out of focus? Photographers do this all the time, to reduce the impact of ugly backgrounds, and to add ambiguity, intrigue or mood to a scene. To begin with, unless you have ver…y expensive equipment, try using a focal length of 70mm or greater. Then set your aperture to its maximum (wide open), and get as close to your subject as you can. If your background isn’t too close to your subject, this, too, will enhance the effect. If you have a fast (large aperture) prime lens, start with an aperture one stop smaller than its maximum, as this will often produce sharper results within the focussed region. Unfortunately, with compact cameras, actual focal length is so small that it is more or less impossible to create a narrow DoF.