I have recently written a post in my sunglasses blog about Pupil Distance and why this measurement is a must when you order glasses on line. A lot of people found this entry useful, so I decided to talk little bit more about other facial measurements that opticians take.
When you order eyeglasses with single vision lenses, the prescription provided by your eye care professional and PD is usually all you need. If you decide to get bifocal glasses, you will also need to know Segment Height measurement for the frame you choose. This measurement determines the height of the reading area of the lens. Why is it necessary to get an accurate seg height measurement when ordering bifocal sunglasses or eyeglasses? Because if the reading segment of the lens is misplaced, you will have to bend your neck backwards or pull the eye glasses down the nose when reading.