Lens equivalents among the formats is often misstated as being scaled up based on the ratio of the format diagonals. The problem with this method is that it breaks down with larger formats because the magnifications become significant. Most people seem to want to know which lens to choose for shooting portraits based on their experience with the 35mm format. Invariably, the answer given to them is the scaled up focal length with a disclaimer that "shorter focal lengths seem to work better with the larger formats, so you can get away with a shorter lens." This is not really true. What happens is that when your magnifications start getting significant (say about M=0.1), the lens extensions become more significant. Your "effective focal length" becomes longer. For example, if you're shooting a 300mm lens at a magnification of 1/3, your effective focal length becomes 300*(1+1/3) = 400mm--this is now, effectively, a 400mm lens--you are getting the angle of view of a 400mm lens. To make things easier, there are 2 tables attached. One is for infinity distance, where the lenses scale exactly with the format size. You'll see that the focal length for a 5 inch format length (4x5 inches "tall" length is 5 inches) is exactly half that of the 10 inch format length (10 inches is the "tall" or "portrait length" for the 8x10 inch format. The other table is calculated for a distance of 5 feet to the lens--this is the standard "go to" portrait length for pleasing perspective. Note that this measurement is TO THE LENS--to the entrance pupil--so the similar cameras with different format sizes will have the lenses positioned at the same position--the same perspective. You'll see that there is quite a difference at 5 feet as compared to at infinity for focal length requirements. For example--you'll see that for a 135mm lens on 35mm camera focused on a distant object, you'd need a 1333mm lens for 11x14 to get the same distance on the 14 inch length as the 35mm camera gets on the 36mm length. However, up close at 5 feet, the 11x14 camera would need only a 746mm lens to get the same field of view, same angle of view, same perspective as the 135mm lens on the 35mm camera. The tables are calculated for common 35mm lens sizes starting with 21mm up to 135mm.