I'm an old school photographer, brought up on 35mm SLR cameras. During those days, I knew that when a lens manufacturer specified that a certain lens is a 50mm lens, it's a 50mm lens. With lenses for dSLR, the focal length specs seem to have changed. For example a dSLR 50mm lens is equivalent to 75mm in 35mm format. However, when it comes to zoom lenses, everything becomes fuzzy to me. For a Zmm - 200mm may mean Zmm - 300mm for one manufacturer, but may mean Zmm - 320mm for a different manufacturer, in 35mm format. As you can see, I can only visualize focal length from a 35mm prospective. Sorry to sound absolutely ignorant, but can someone tell me how focal length for dSLR lenses are calculated? And how I can work out the 35mm format equivalent, so that I can get a handle on this fuzzy problem of mine. Many thanks in advance.