Unless you’re going for a look that says, “Kick me, I’m a nerd,” you probably want a pair of glasses with the thinnest lenses possible.
But if you have a strong prescription and you’re ordering your glasses online, figuring out how to get the thinnest lenses possible can be perplexing.
Not to worry.
We’re going to make it easy for you.
All you’ll need is your prescription and a handy online tool that will estimate how thin your lenses will be.
This handy online tool is the Lens Thickness Calculator. It can be found at the OptiCampus website.
Before we talk about how to use the Lens Thickness Calculator, let’s look at the factors that can thin your lenses:
- Your prescription. This you cannot change to get a thinner lens. Doing so would be counterproductive, in that you would not be able to see well if you take it upon yourself to lower your prescription below what your eye doctor has prescribed.
As a general rule, single vision prescriptions result in thinner lenses than multifocal prescriptions, all things being roughly equal. Multifocal prescriptions are those with an NV-ADD (Near Vision reading ADDition). Having an NV-ADD, or just an ADD, as it’s usually written, enables you to get bifocals and progressive glasses.
In addition, prescriptions with astigmatism corrections – if you have this, there will be numbers in the Cylinder (CYL) and Axis (sometimes abbreviated X) fields – generally result in thicker lenses than those without astigmatism corrections.
- Your lens index. The lens “index” is short for the lens’s “index of refraction.” That means how much the lens bends the light that enters it. That’s what refraction is. Bending the light focuses it right into the middle of your visual sweet spot, the retina, so you can see well.
You can change the index to thin the lens, up to a point. After that point, the law of diminishing returns kicks in. The lens that is recommended for your prescription will be the thinnest lens. Choosing a lens index that’s higher than the recommended lens index won’t make the lens thinner, and you’ll end up paying more for no improved result.
If this is confusing, don’t worry. We’ll clearly explain where the point of diminishing returns shows up when we look at the various lens indexes. In addition, you’ll be able to see it too, when you start playing with the Lens Index Calculator.
- Your lens width. This can have a considerable effect on your lens thinness. Once you have settled on the correct lens index, having determined that going to a higher lens index number won’t result in a thinner lens, the lens width (also known, weirdly, as the “eye size”) will be the last, and in some situations, the most important factor in thinning your lens.
In part 1, we outlined the factors that can thin an optical prescription lens: the index of refraction and the lens width, also known as “eye size”.
Now 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:
SPH -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.
Now we’re really going to see the law of diminishing returns at work when we go up a notch, to the 1.61 high-index single-vision lens. Here are the results of this lens:
CT: 1.5 mm / ET: 3.5 mm
No change, and you’re now (theoretically) spending $19.95 on a lens that isn’t any thinner than the 1.59 pure polycarbonate lens that’s $9.00, and is only a tiny bit thinner than the 1.57 mid-index lens that’s free.
But even after you’ve selected the lens index that will give you the thinnest lens, you can still thin the lens further, if you narrow its width.
Let’s lower the lens width to 48 mm and see what happens with each lens. First, the 1.50 lens:
CT: 2.0 mm / ET: 4.1 mm
The Edge Thickness is a tiny bit better, 4.1 mm as opposed to 4.4 mm on the 50 mm-wide lens. How about the 1.57 lens?
CT: 1.5 mm / ET: 3.3 mm
Now we’re talking! We’ve shaved half a millimeter from the center and almost a whole millimeter on the edge, just by lowering the lens width.
How about with the 1.59 lens?
CT: 1.5 mm / ET: 3.2 mm
No change on the center, only one tenth of a millimeter on the edge. Stand and salute the diminishing returns on parade!
Let’s take a quick look at the 1.61 lens, just to make sure it doesn’t make the theoretical eyeglass lenses any thinner:
CT: 1.5 mm / ET: 3.2 mm
That would cost $10.95 more for no improved result.
So unless you want a pure polycarbonate lens for its impact-resistance feature, you’re wasting your money by spending $9.00 to get a higher-index 1.59 lens that won’t be any thinner than the free, 1.57 mid-index lens.
But once you’ve confirmed which lens index that will result in the thinnest lens for your prescription, and you want to thin the lens even further, play around with the lens width on the Lens Thickness Calculator. Go as low as you can and still keep the lenses wide enough for a comfortable field of vision and attractive look.
For example, the 48 mm wide lens may be just as narrow as you can go without making the glasses look too small. But if you can narrow the lens even more, you will see that the lens will continue to become thinner.
If you do check out the Lens Thickness Calculator to see the estimates of different thicknesses that different lens indexes and lens widths yield, keep a few things in mind: When you are selecting the Lens Material Type, the categories listed on the Lens Thickness Calculator don’t exactly correspond to the lens index categories on our website, although they are basically the same.
For example, the Hard Resin lens is the 1.50 lens. The 1.56 mid-index is essentially the same as the 1.57 mid-index. The polycarbonate lens is the 1.59 mid-index lens. All of the 1.60 lenses on the Lens Material Type list are the equivalent of the 1.61 high-index lens.
So to recap, stick with the lens that our website recommends when you enter your prescription. That will get you the thinnest lens for your prescription, if you also select the lowest lens width that will also work well for your face size and shape.