The Optimal View
Similar to clothes, your vision can be tailored to achieve the best fit. When the visual assessment portion of the exam is performed, optometrists use a phoropter to derive your prescription. You may remember sitting behind one of these over your eyes, answering "1 or 2? 3 or 4?" as the letters get smaller and tougher to read. What your optometrist is doing is discovering your most accurate prescription.
At the end of it all, you're given your prescription as per the doctor's findings. Now on to getting your glasses!
The Vertex Distance
The phoropter typically sits 12 millimeters in front of your eyes during the exam. However, not every pair of glasses sit the exact same distance. Some sit closer, some sit further depending on your nose bridge, the glasses' nosepads, and even the thickness of the plastic or the width of the bridge. There are many factors which affect how far the glasses sit in front of your eye, and it can vary from 12 millimeters.
What does this mean? With your prescription, any little change in the vertex distance alters the vision you get through the lenses. While lower prescriptions will not see a noticeable difference, higher prescriptions will definitely notice a difference in intensity of their prescription. This is something that should be taken into consideration when choosing a frame and the corresponding lens.
The Pantoscopic Tilt
Again, we refer to the phoropter to make this adjustment. The phoropter usually sits perfectly straight, parallel to your face. Again, not all glasses will sit the same way. Most glasses have something called a "pantoscopic tilt", which causes the frame to slant inwards towards your cheek. Both an aesthetic design and a comfort factor, this will also affect the vision through the lens. This angle usually varies between 2° and 12° inwards.
Frames that tilt the other way, away from the face, are known as having a "retroscopic tilt", although this is a less common fit type.
The Frame Wrap
In the most extreme cases, frames will have a high wrap (in the case of sunglasses). Most optical frames will have a mild curvature to them, making them both a more comfortable fit and a little more natural looking.
The phoropter is again set at a perfectly straight angle. Changing the angle of the frame can begin to create a fishbowl effect, not to mention distortion similar to a funhouse mirror. Compensating for this wrap can help to alleviate these distortions. These angles vary from 2° to about 10°, but some sunglasses can wrap up to 30° in the most extreme cases.