The pupil of the human eye can dilate to 7mm. If the exit pupil of the scope is 7MM, your eye can use all of the light transmitted. If the exit pupil is less than 7MM, the scope is not illuminating the eye to the full extent. A 7MM exit pupil is the holy grail. Exit pupil is calculated by dividing the diameter of the objective bell (front lens) by the magnification Thus a 4x28 scope has a 7MM exit pupil. The same for a 6X42. A 3-9X42 will only achieve 7MM at 6 power. Some big variables have 56MM bells. This gives a 7MM exit pupil all the way to 8 power. On a sunny day, this isn't very important, but in low light situations, the larger the objective bell, the better you will see the target. This is only one of several considerations when choosing a scope. But it cam be a very important one.