One of the mysteries of photography is Depth Of Field (DOF). It combines various factors, including camera to subject distance, focal length of the lens and aperture in use. You calculate all of the above and know what will appear sharp and unsharp in the image. Those who use fixed focal length lenses have had the advantage of having a DOF scale on the lens, which yields approximate zones of sharpness when the focusing distance and aperture could be sighted simultaneously within certain color brackets that might or might not be etched on the lens barrel. Those with zooms were out of luck in that regard. I do remember an old Nikkor zoom that attempted to have DOF scales etched using various colors and designs, but it was all quite unreadable and while interesting, useless.

You can also depend on the DOF preview function in your camera, but this becomes less than helpful at narrow apertures, and is always, at best, an approximation of results. But it is amazing how many photographers don't even know that exists, or what it possibly could be used for. And Canon's DOF Exposure mode has always puzzled me; it can only work when presented an easily solvable problem and seems not to object when subjects at different distances are out of range.

DOF calculations are, after all, just that. You add A+B and you get C. But having to do equations and such in the field is absurd; it's more the subject for an RIT midterm. One product that recently caught our eye in this regard is not a new product, but one that's been updated for the digital age: the ExpoAperture2 DOF Guide.

The DOF Guide is a flat, circular, and interlocking set of dials that boil down all the formulas to a matrix of numbers. It's an analog calculator that has a series of what-if relationships. For example, what if you are shooting with a 35mm camera and a 45mm lens at a subject 6 ft away. And what if you want a shallow depth of field, say from 5-7.5 ft. Well, match the dials as directed and the answer appears above a line reading f/8.

While at first the DOF Guide might seem archaic (and for old-timers something they have seen before, the Wallace DOF Guide), this updated version incorporates all the latest format conversion factors, including the latest digital cameras (identified by their "crop factor" or the relationship of the sensor size to 35mm format), as well as formats larger than 35mm. In other words, this is a DOF Guide for the digital age that relies on a decidedly analog design and read-out.

About the size of a CD, the DOF Guide is printed on very tough coated material that looks like it will take some use in the field, although using the supplied slip-in case for it when dormant in your kit is probably a good idea. At first it seems to be a blizzard of numbers and spins, but working with it once or twice will clear that up. And the makers include an instructional CD and a digital camera "crop factor" guide that helps you define the sensor size, thus the coverage of the image circle and subsequent DOF effects for your camera. If your camera does not appear on the list it is a simple matter to find the sensor size (such as APS-C, for example) and then just use that in your calculations.

You can use the calculator in various ways, including setting an aperture and then finding the resultant depth of field, or plugging in the "desired" depth of field and then finding the aperture. All the variables come into play and using the "what-if" advantages of the calculator allow you to decide which lens or subject distance will do the job for you.

Does the ExpoAperture2 DOF Guide provide information that you can't approximate with your current setup? I guess approximation is the operative word. The DOF Guide is designed just as much as a field tool as it is a teaching tool, and I found myself playing with it just to make some what-if calculations on various format and lens combinations. It is for those who want to make precise DOF decisions quickly and easily, and not worry about the fact that they can't quite see the finder when activating their DOF preview function. And it is a very clever item indeed.

ExpoAperture2 sells for $29.95 plus shipping, etc. at www.expoimaging.net or you can find a dealer using their Dealer Locator at the same website.

For more information, you can also write ExpoImaging, Inc., 121 Aviation Way, Watsonville, CA 95076, or call (831) 761-2040.