Eye relief for scopes is generally just given as a single, static figure, such as 3.5 inches. In reality, there is a window around 3.5 inches where you'll be able to get the full field of view through the scope; say, from 3.3 - 3.7 inches, for example. My question is, why are some scopes more forgiving in this respect than others? With variable power scopes, as you crank up the power does this window shrink? Does objective size, tube diameter, number of lenses, or any other easily quantifiable parameter affect this?