Re #5: I've read that plenty of times, but it doesn't help because it's vaguely clueless. A 25mm or 28mm focal is pretty standard on an SLR with a 50mm lens, which is why I'm guessing a 50mm lens. The focal, lens size, and exposure size are all different things. The common '35mm' associated with an SLR describes the size of its negative. These usually, but not always, have 50mm lenses. The effect of a focal length changes with the size of the lens, because with a larger lens the same focal length forms a differently shaped triangle. This is how you can adjust the depth-of-field with a manual SLR simply by changing the F-stop, which is the aperture size. By changing the aperture size, you're changing the amount of the lens that is used, which is effectively almost the same thing as changing the lens size itself. This changes the shape of the light-cone, which directly determines the depth of field. However, if a 50mm focal is supposed to and is designed to approximate the depth-of-field of a human eye, then it can be extrapolated back based on the lens aperture size and focal length of a human eye. That is, if they're right.