in focus: defining depth of field
30/09/2008
I’m having a lot of difficulty understanding depth of field. Can you explain it more clearly? Peter, Seoul.
Depth of field (DoF) is difficult to explain without diagrams and charts. It’s more about math and physics than it is about photography. However, I can give you some starting information, which is enough for most photographers.
DoF, simply, is the distance between the nearest and farthest objects which appear to be in focus. This is quite a subjective concept, because it’s difficult to always define the focal range. Further, all photographers have different views regarding the use of DoF.
However, DoF is extremely important to photography and film.
Focal range is controlled by the camera’s aperture. A small aperture (eg f22) will have a very wide depth of field. A small DoF will result in large elements of the photograph appearing in focus. Conversely, a wide aperture (f2.8) will have a much smaller depth of field. As a result, the out of focus area will be quite big. Yes, it’s ironic that these concepts seem inverse (small aperture - wide DoF, wide aperture - small DoF).
When it comes to understanding what parts of the photo will be in focus, there’s a two thirds rule applied to DoF. One third of the area in front of the focal point will be in focus, but two thirds of the area behind the focal area will appear to be in focus. So it’s important to know that much more of the focal range is behind the focal point, not in front. It’s not evenly split (50/50) around the focal point.
There are a few other factors which can make the focal range either larger or smaller.
Firstly, as stated, small apertures give wider DoF.
Second, shorter lenses also create a wider DoF. A 50mm lens will give a much wider DoF than a 200mm lens, if the same aperture and settings are used.
Finally, the greater the shooting distance, the greater the depth of field. This one is logical really. If photographing mountains, DoF can be read in kilometers. If photographing bugs (macro photography) DoF is mere milimeters. So the further away your subject, the longer the DoF.
Generally, photographers have two approaches to controlling DoF. In most cases, photographers prefer short DoF and large areas which are out of focus. This helps control the viewer’s attention by blurring the foreground and background elements, while keeping the subject in perfect focus. Portrait photography is a good example of this. Most portrait photographers use longer lenses and wide apertures.
Small DoF also creates more ‘bokeh’ which is one of photography’s most important attributes.
However, there are many times when a wide DoF is desirable, such as in landscape photography. A good landscape photo should appear to have everything in focus, from mountains at the back to rocks in the foreground. This can be achieved with wide lenses, small apertures and focusing to infinity (which all cameras can do).
Those are the basics to depth of field. The important thing to remember is that DoF is easy to control because the aperture, lens length and focal length all play a simple part. Combine them well and you can be a master of controling DoF, producing great photos with very high impact.
Happy shooting!
David Smeaton
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