One of the fundamental tenets of the physics of photography is the fact that longer lenses (more telephoto, I mean) produce a shallower depth of field. But this "fact" is, in fact, not accurate. I know, I’ve written about this principle, as has every other photography writer who has ever put pen to paper. But the fact is the physics just don’t hold up. What is true is that the APPEARANCE of depth of field shrinks in telephoto pictures, because of the compression of a scene. If in fact you simply use a telephoto lens to "zoom in" to a particular part of a scene, and keep the elements in the scene the same relative size, then the physical measurements of the depth of field will not change. That’s right: you can produce the same depth of field with a 19mm lens as you can with a 200mm lens. If you want to understand how, Peter West Carey from DPS has pointed out a great little article about this phenomenon, written by Matt Brandon of The Digital Trekker. Sure, you can still strap on a long lens and create the EFFECT of shallower depth of field in most cases, but for a complete understanding of the physics, have a look at this link.