The back focus distance - distance from mounting ring to image plane - is fixed for a given lens, you can change it a bit but the focus ring would need to be reset and the actual mechanism doesn't allow much adjustment.
That means, if you can use "film" lenses on a digital camera, back focus is the same for digital or "film" lenses. Most of the major film camera makers kept the back focus and the mounting ring the same so we can use our old lenses with the new cameras. Saying "50mm doesn't equal 50mm" isn't exactly true. A 50 is a 50 is a 50. The "50mm" means the image size at the film plane is the same as if you put a pinhole 50mm from the film plane.
A "normal" lens is the lens that gives an image that looks "normal" to us - our eyes see the world about the same. A shorter or longer lens looks odd, you can tell it's a wide angle or telephoto shot. For 35mm the "normal" lens is 50mm or so, so usually you get that lfocal ength first or at least a zoom including it. For Hasselblad type 6x9 cameras somewhere around a 85mm lens is "normal". For an 11x17" view camera the "normal" lens is a long focal length indeed!
The lens that is "normal" for a digital camera is different if the imager size is not exactly the size of a 35mm film frame. Most imagers are smaller, so the "normal" lens is shorter. A 50mm lens is a slight telephoto in this case. A high quality "film" lens is still a high quality lens, and will give excellent results with a digital camera. You just have to adapt your use of the sizes you're familiar with if you were a 35mm photographer.
