Depth of Field (or as people seem to call it these days, Depth of Focus) isn't just for widgets. Photographers spend hours discussing it and while most of the discussion is pretty academic, in all senses of the word, it's an important topic for photographers to understand. For most people, this simply means knowing what to excpect from your equipment. I won't say I've never heard anyone talk about the DOF numbers of a particular lens as they considered purchasing it, but I will say I've never know anyone to actually make a purchase based on those numbers. At the end of the day, DOF just isn't as important to most people as focal length and price.

For many of us, though, that may be about to change. If you've been shooting digital for a while, you've probably noticed two different phenomena, which may seem contradictory. On the one hand, you may have noticed that DOF is narrower than you might expect when you use lenses designed for your DSLR. On the other hand, you've almost certainly noticed that DOF is much larger.

DOF is a tricky concept to understand under the best of circumstances. Because it is defined as the area of an image that isn’t unacceptably out of focus, it is realted to format size, format resoultion, and print/display resolution and size. It also depends a great deal on how good your eyes are: for most people, an image begins to look fuzzy when the dots of ink or dye in a print are larger than 25µm-35µm, a value known, appropriately enough, as the Circle of Confusion. For those of you who are interested in the underlying math, both Paul van Walree and Bob Atkins have informative articles on DOF in general and how it plays out with DSLRs. For the rest of us, though, there are a couple of rules of thumb:

• For an equivalent field of view, the DSLR DOF over most of the range will be multiplied by about the “digital multiplier” for your camera.

• When using the same lens at the same focal distance, the DSLR DOF range for most distances will be narrower than a corresponding 35mm shot, by about the “digital multiplier for your camera.

• Close to the Hyperfocal distance, the DSLR will have much more DOF than a corresponding 35mm shot, in some cases from the near focal distance to infinity.

• Most digital macro work will exhibit less DOF than you would expect compared to 35mm, in many cases no more than a few mm.

This makes digital absoultely ideal for many purposes. Landscape photography, for instance, is a cinch. When taking protraits, though, life becomes more interesting. It’s much more difficult to control the background blur with the lens, even at high f-stops, and it’s much more likely that blur effects are going to need touching up in Photoshop.