In photography, the idea of depth of field, or the portion of a photograph that is in focus, isn't an easy one to grasp. Like almost everything else in the photographic world, it is possible to figure out what the depth of field of a given (or even hypothetical) photograph based on the size of the film or imaging sensor, focal length, aperture, and the distance to your subject. Given the number of factors involved though, the math gets a little complicated. Thankfully, the folks at ExpoImaging have created the ExpoAperture2 Depth of Field Guide to figure it all out for you.
The ExpoAperture2 comes with two depth of field calculators--think of them like circular slide rules--one that covers wide and normal focal lengths (15-135mm) and one that covers telephoto focal lengths (70-600mm). The kit also comes with small plastic sleeves to store the discs, a reference chart to help you figure out the size of your camera's sensor or film, and a user's manual on CD. The manual includes a very good explanation of depth of field and how and why you'd want to control it. The result is a deeper understanding of photography that should benefit any but the most accomplished photographers.
If you don't own an SLR, then the ExpoAperture2 won't provide as much practical information, since the calculators rely on the actual focal lengths of SLR lenses rather than the equivalent focal lengths to which most compact cameras refer. Also, the calculators don't include the small sensor sizes that most compact cameras use. However, that doesn't do anything to diminish the fact that the ExpoAperture2 is one of the best depth-of-field learning tools I've come across in a long time. The ExpoAperture2 is available now for about $30.