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Basic Explanation of Depth of Field.
The term ‘Depth of Field’ in photography relates to the area of an image that appears sufficiently sharp that it can be called sharp. All other areas are, to a lesser or greater extent, blurred. Fortunately, depth of field is something we can - and should - control. Although it is a technical subject, depth of field should be considered as a tool to be used as part of composition.

If you are photographing a traditional landscape, a deep depth of field is usually selected. If you are shooting portraits, a shallow depth of field is often selected to isolate the subject from the background.

The main influences on depth of field are:-
  1. Lens aperture:The smaller the aperture (the larger aperture number), the deeper the depth of field is. Conversely, the larger the aperture (the smaller aperture number), the shallower the depth of field is.

  2. Camera to subject distance:The effect of camera to subject distance is that the closer the camera is to the subject, the shallower the depth of field becomes and vice-versa.

  3. Focal length:Focal length affects the depth of field, but perhaps in a less obvious way than may be at first imagined. For the purpose of basically understanding the influence of focal length on the depth of field, the depth of field gets shallower at any given distance as the focal length increases. BUT what happens is, if the subject size is kept the same on the image (i.e. the subject to camera distance is increased as the focal length of the lens is increased), the depth of field stays very nearly exactly the same.
Miscellaneous related information and tips:-
  • The term for isolation of the subject using a shallow depth of field is ‘differential focus’.
  • The size a picture is printed at affects the depth of field too,so expect the depth of field to get a little shallower if you print a picture really large. You may notice it if you have only just caught a persons face in the rear of your depth of field on your screen. Blowing the picture up may move the subject just outside of the depth of field - losing you those sharp eye lashes maybe.
  • There is a term in photography called ‘bokeh”, given to the quality of the blur in the unsharp areas in the image. It is used to refer to the effect of shallow depth of field that creates a blur of colour behind the subject. The depth of field plays a large part in determining how out of focus the area is and whether or not it creates a pleasing effect in the image is largely down to the design of the lens – a good bokeh looks creamy.
  • Depth of field also varies with the sensor size of the camera used, but to a lesser extent. For example - if you use a camera phone with a tiny sensor, the depth of field will be deeper at any aperture (all other factors being equal) than on your DSLR. You will never achieve differential focus with a small sensor – in fact, even if you shoot a crop sensor DSLR you will notice a difference in the bokeh between it and a full frame DSLR.
  • The depth of field is not always half in front of the focal point and half behind, but varies from lens to lens and can be as extreme as a quarter in front and three quarters behind – it is usually around a third in front and two thirds behind though.
  • If shooting portraits, be careful not to use a too shallow depth of field or you will have to choose which eye to have in critical focus! Try f2.8 to start with.
  • Most lenses are best not used at the extremes of aperture settings because wide open they often have their worst sharpness and chromatic aberrations, and when used stopped down below f11 or f16 can lose sharpness due to diffraction. The sweet spot for most non-professional grade lenses is around two to three stops below maximum so for an f1.4 it could even be f2.8 or f4 before it is sharp, sharpness peaking at f5.6 or f8 then it usually starts to drop off again after f11.
  • Depth of field is particularly relevant to you if you want to shoot close-up or macro whereby you should always aim for the deepest depth of field – minimum depth of field would be f11 here, True macro lenses perform better close to the subject and it is OK to use smaller apertures (f22 or even f32) with these lenses.
If the above has whetted your appetite to learn some of the details behind depth of field, a good place to start is:


Also, this chart is a useful pictorial explanation of F-Stops.

DOF chart

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