Three factors play in: - where the subject is looking. Make sure they are looking at leat 4 or 5 meters out, not at you taking the picture! Your pupils get sli

Three factors play in: - where the subject is looking. Make sure they are looking at leat 4 or 5 meters out, not at you taking the picture! Your pupils get slightly closer and further depending where you look - where the ruler is. Make sure it is at the same level as your eyes, in terms of distance from the back/front of your head. Make sure the ruler is as close to 90 degrees to the nose as possible, as parallel to the eyes as possible - make sure the camera is as parallel to the ruler as possible Also I find printing out one of the pic (just b&w on a regular laser printer) can be easier to measure, as opposed to do it on a screen.