The final consideration that can affect the overall performance of a pair of glasses is the measurements. When glasses are fitted, two measurements are taken: the location of the bifocal, and the distance between the pupils.
Sometimes the pupil distance is measured by the doctor, but more often the optician who fits your glasses will do this herself. Be sure to hold your head steady and look exactly where asked. If this measurement is wrong, it can lead to headaches and eye strain.
The most important measurement is for the bifocal, and the most common reason this measurement is wrong is due to posture. Most opticians will measure this with a marker and a small ruler marked in millimeters. It is important to sit in a normal posture, not unnaturally straight. Hold the chin at a normal level. Trying to sit up straighter than normal or holding the chin too far forward can mean bifocal lines that are set too high or low, making the glasses difficult to use. Sit naturally, and ask to have to measurement re-taken if there are any doubts.
If for any reason the glasses don’t perform the way they need to, bring them back to the retailer. Most reputable optical shops will fix the problem at no cost, within a reasonable amount of time—usually less than sixty days from the original purchase.