Online Tutorials: Depth of Field
Depth of field can be thought of as the amount of the image which is in focus. This means that either side of the selected point of focus, there is a region in which the image remains in focus. Moving outside of this band of focus (towards or away from the lens), the image becomes progressively out of focus therefore less sharp.
The amount of depth of field is controlled solely by magnification and aperture. Since the magnification is normally fixed for a given subject, the depth of field is usually controlled by aperture alone. Wide apertures (such as f2, f2.8) give less depth of field whilst narrow apertures (such as f16, f22) give much more depth of field.
Telephoto lenses always have a shorter depth of field compared to wideangle lenses.
For example a 70mm lens using an aperture of f16 will have a depth of field of 361.7m at 10m distance whereas a 400mm telephoto lens with the same aperture and distance will have a depth of field of 58cm. This is useful when you do not want any background distractions at sporting events or wildlife shoots.
50mm Macro lens at mimumum focus distance of 24cm with the following three aperture sizes
f2.8, Depth of Field = 0.21cm
F13, Depth of Field = 0.98cm
F32, Depth of Field = 2.34cm
Depth of field can be seen clearly in these images.
The three images were shot using the same focal length at different aperture settings. You can see that the top image has a very shallow depth of field and the bottom image has a longer depth of field that includes the background leaves.
Your choice of depth of field should depend on your subject and choosing the appropriate F-Stop can make or break an image. Some cameras have a depth of field preview button to allow the photographer to see what the current F-Stop's depth of field is going to show.
Depth of field can also be controlled using neutral density filters. These cover the front (or back) of the lens and allows the amount of light to be sufficiently controlled to allow a smaller F-Stop and shallower depth of field for a given shutter speed. This may be needed if your camera cannot handle the high light levels of the subject using a large aperture.