The first thing to look at before looking for lenses is your prescription. Using the concept that 0.00 means no prescription and +0.25 and -0.25 are the lowest prescription possibilities, it's safe to conclude that prescriptions ranging from -6.00 to -12.00 are considered high, as are +3.00 to +6.00. The higher your prescription, the more important it is for you to order thin lenses. Thinner lenses cut down on the bulk and ghost images that occur in thick lenses. Plus, aesthetically, thinner lenses look better in frames. If your prescription is marked with a plus (+) in the sphere (SPH), your lenses will be thicker in the middle. Should you prescription have a minus (-) in the sphere (SPH), your lenses will be thicker at the sides. These are all things to consider before buying glasses online.
Thinner lenses are also called high index lenses. The most common lens is the CR39 plastic lens and it has an index of 1.5, making it great for low prescriptions like those that fall into the range of -2.00 to +2.00. If your prescription falls outside of that range, it's time to take the next step and go for high index 1.56 lenses. They're typically 25 percent thinner than standard CR39 lenses and can decrease the thickness for prescriptions ranging from -4.00 to +2.00.
Moderate prescriptions that range from -6.00 to +4.00 may be considered as a high prescription by some eye care professionals. While it's at the low end of the high spectrum, it will definitely leave your lenses feeling thick and bulky, adding extra stress on your nasal cavities. Prescriptions in this range are generally ineligible for CR39 basic lenses. The best lens for this prescription is the 1.61 high index that actually cuts the thickness down by 30-50 percent, depending on where your prescription falls on the spectrum. Prescriptions falling at the high end of the -6.00 to +4.00 spectrum can also use high index 1.67 lenses, as can prescriptions ranging from -8.00 to +4.00. They drastically reduce the thickness of high prescription lenses.
Another option in thin lenses works best for prescriptions ranging from -12.00 to +4.00. The ultra thin 1.74 high index lenses will require you to add an ultraviolet protective coating for better protection. These lenses are the thinnest and lightest lenses available on the optical market and should not be used for low to moderate prescriptions due to the warping effect that the lenses have.
Once you understand which prescription range you fall into, fitting your proper lenses into your eyeglasses frames shouldn't be so tough. It's good to know that if your lenses are too thick, they will not fit into your frames, so matching your prescription glasses to the proper lens is important. Additionally, if your prescription is high, opt for the higher index lenses - your eyes and your nose will be thankful.