To your chagrin, you discover that you need reading glasses. As you speak with your friends about their reading glasses, most of the conversation focuses on color and design.  However, you have more pressing concerns about the actual functionality of the lenses.  More specifically, what are the differences between diopter lenses and prescription lenses? 

Similarities: Ready-made Reading Glasses and Prescription Eyeglasses
Surprisingly, there are similarities between the lenses of diopter and prescription eyeglasses. Both lenses can adjust the focus of the natural eye to enable to owner to see clearer.  The lenses also share the same numbering system for its corrective powers, but the slight difference is that prescription lenses have the numbers embedded, while most diopter lenses have the numbers placed on stickers or labels.

Nevertheless, the corrective numbers follow the same rules. If the numbers have a negative sign, the lenses are meant for people who are nearsighted and need help focusing on objects that are far away. If the numbers have a positive sign, the lenses are meant for people who are farsighted and need assistance seeing objects at very close distances.

Major Differences between Prescription and Ready-made Lenses
There are, however, several major differences between the lenses of diopter eyeglasses and prescription eyeglasses.  Generally speaking, prescription eyeglasses can cater to different corrective needs between the two eyes, as well as assist with astigmatism.  Ready-made reading glasses are much less customized. 
Most diopter eyeglasses will have the same focusing adjustment or magnification power on both lenses. The two lenses are typically exactly the same in their corrective powers.  

On the other hand, prescription eyeglasses may have different corrections on each lens. Subsequently, this is why there are letters found on the lenses of prescription eyeglasses.  The OS (which is the abbreviation for the Latin term oculus sinister) refers to the left eye, while the OD (oculus dextrus) refers to the right eye.

The second difference is the prescription lenses’ ability to help eyes suffering from astigmatism.  Thus, the lenses of prescription eyeglasses will have additional numbers, referring to the axis of astigmatism in the eye.

A person who does not have astigmatism, but simply needs help in focusing, will only need the lenses found in designer reading glasses.  However, should you have astigmatism, or your eyes have different corrective needs, then prescription lenses may be ideal for you.