The depth of field (DOF) represents the distance between the nearest and the farthest objects in a scene that are perceived as sharp in the final image. It is one of the most creative effects available to photographers at the time the picture is taken. The whole theory behind DOF is probably beyond the scope of this post but there is plenty of literature for those interested in optics and the science of image capture.
The DOF is dependent on the aperture and the focal distance of the lens (although the image size and viewing distance can influence our perception of the depth of field). The DOF does not jump abruptly from sharp to unsharp, but occurs as a gradual transition (similar to what happens in the eye of an elderly person that lost the ability to focus properly; with a bit of effort we all can teach ourselves to become aware of DOF).
The hyperfocal distance (H) is a term closely related to DOF that represents the closest distance that the lens can be focused while keeping the objects at infinity reasonably sharp. At this distance, the DOF is at maximum – i.e. all the objects between H/2 and ∞ (infinity) will appear sharp for a typical viewer.
Different cameras are characterized by different values for H corresponding to different apertures and focal distances. The following table summarizes some typical values for Canon EOS DSLR families that use an APS-C sensor (e.g. 300D, XT, XTi, XS, T3, 40D, 50D, etc.):
f (mm) F-Number H (m) H (ft)
17 4 6.44 21.13
17 5.6 4.60 15.11
17 8 3.23 10.59
17 11 2.35 7.72
33 4 24.23 79.50
33 5.6 17.32 56.82
33 8 12.13 39.81
33 11 8.83 28.98
55 4 67.28 220.73
55 5.6 48.07 157.71
55 8 33.67 110.45
55 11 24.50 80.38
85 4 160.64 527.04
85 5.6 114.77 376.53
85 8 80.36 263.66
85 11 58.47 191.83
As we can see, the smaller the focal distance and the smaller the aperture, the smaller the hyperfocal distance is. While the changing the focal distance may not be always desired (we will analyze this later), the aperture can be changed in most cases.
Working at the maximum DOF is probably desired by most in landscape photography. Let’s consider a Canon 40D camera with a fixed focal lens of 17mm (or with a zoom lens set at 17mm). According to the table above, for F11, all the objects between H/2 = 1.18 m (3.86 ft) and ∞ will appear sharp.
Please note that the table used in our example does not apply for all cameras; I will return later with an in-depth discussion of DOF and the theory behind it. But if you are curious right now, please go to DOFMaster web site for a detailed discussion and a lot of useful tools. Some theory is also available on Wikipedia (search the terms discussed here).
According to the above, some may be tempted to use even lower apertures (e.g. F16, F22 or even F32) in order to decrease the hyperfocal distance and, therefore, increasing the DOF. To their surprise the results may be bitterly disappointing. I will return with some examples in my next post.