We’re going to look at a prescription to see how different index numbers can make the lenses thinner. We’ll change the lens width, remember, that’s also known as the “eye size” (even though it doesn’t refer to the size of your eye), to see how this affects the thinness of the lens.
To do this, we’re going to use a fairly common prescription. Most eyeglasses wearers are nearsighted, which means that the number in the Sphere (SPH) section on the prescription will be minus.
Most people who have an SPH correction also have an astigmatism correction. So we’re going to create a common prescription, with an average Pupillary Distance (meaning the distance between your pupils measured in millimeters, PD for short), 63; an average lens width (all together now: also known as “eye size”), 50 mm; and an average bridge (that’s the part of the frame that goes over your nose), 18 mm.
Here it is:
PH -2.00, CYL -1.00, AXIS 90, PD 63, Lens Width 50, Bridge 18.
(For the sake of simplicity, we’ll assume that the prescription is the same for both eyes.)
Let’s put these numbers into the Lens Thickness Calculator and see what we come up with when we use different lens indexes.
With nearsighted prescriptions, the lens is concave. With farsighted prescriptions, the lens is convex. A concave lens is thinner at the center, thicker at the outer edge. A convex lens is the opposite.
You will see then, that with nearsighted prescriptions, the Center Thickness of the lens will be a lower number than the Edge Thickness.
Here’s what our formula yields with the 1.50 standard-index lens, which comes free on an order of single-vision glasses:
Center Thickness (CT): 2.0 mm / Edge Thickness (ET): 4.4 mm
Nice and thin at the center, but maybe not as thin as it could be at the outer edge. Let’s try it with the 1.57 mid-index lens and see what we get:
CT: 1.5 mm / ET: 3.6 mm
That makes the lens thinner at both the center and edge, and the lens is still free. Now let’s try it with the 1.59 pure polycarbonate single-vision lens, which costs $9.00, and see what happens:
CT: 1.5 mm / ET: 3.5 mm
You’re spending $9.00 for this lens, which is not a huge amount, but this slightly higher index doesn’t make the eyeglasses lens perceptibly thinner than the free 1.57 lens does. Now we’re starting to see the law of diminishing returns come into play.
However, even if the lens would be just as thin with the 1.57 lens, it would be worth it to spend the $9.00 on the 1.59 pure polycarbonate lens if you’re getting rimless glasses or if you’re buying glasses for kids who can be expected to treat the glasses roughly.
That’s because the 1.59 lens is a more impact-resistant lens than the 1.57, which is still a fine lens for people who treat their glasses with care.