How to Choose Thinner Lenses

Why are some lenses thinner than others, for the same prescription?

All things being equal, the thickness of your finished lenses depends on the refractive index of the physical material from which the lens is made. The higher the refractive index number, the more strongly the lens material bends light, and the thinner your lenses will be. These are the approximate refractive index values of a few materials:

Material	Index	Comments
Water Ice	1.3
Human eye's lens	1.4
CR39	1.5	Standard plastic spectacle lens
Crown Glass	1.5	Standard glass spectacle lens
MR8	1.6	Thin plastic lens
MR10	1.67	Superthin plastic lens
MR174	1.74	Ultrathin plastic lens
Thinnest glass	2.14	Thinnest possible glass*
Diamond	2.4

* The highest index currently available in a spectacle lens is 1.9.

If you were to make a lens out of pure diamond it could be very thin indeed, not to mention virtually scratch-proof. Fortunately there are some much cheaper alternatives that can still provide you with impressively thin, light lenses.

In the real world, how much difference does lens material really make?

To show the difference made by changing only the refractive index of a lens, we made four pairs of identical specs to a -6.00D prescription. In each case, everything is the same apart from the lens material. These are actual lenses in real spectacle frames, not a simulated demonstration:

1.5    -    1.6    -    1.67    -    1.74

As you can see, in the real prescription lenses we've fitted here, the 1.74 index lens is less than half the thickness of the standard plastic lens. Also, because there's less physical lens material, the thinner lenses are much lighter and more comfortable to wear, especially if you wear a high prescription. 

So which should you buy?

Because you see the most benefit from high index thin lenses with the higher prescriptions, we recommend you consider choosing lenses as follows:

Prescriptions above 2.00D - 1.6 index
Prescriptions above 3.00D - 1.67 index
Prescriptions above 4.00D - 1.74 index