The digital device that captures the image on a digital SLR is about 2/3 the size of a 35mm film frame. As a result, you effectively get a 50% increase in the focal length of your lens when using a traditional film lens on a digital camera body. This is good, at the long end, because your, say, 300mm lens becomes a 450mm lens; on the other hand, your perfectly good wide angle 28mm lens is now a 42mm lens, nearly a portrait lens. So plan ahead if you're expecting to get mileage out of your 35mm lenses on a new digital body.