One thing that many people notice after cataract surgery is near instant improvement in vision. But with typical cataract surgery, the distance vision becomes clearer than the up close reading vision. Reading vision can remain blurry and require reading glasses to correct. But what is the optimal power of those reading glasses?
For most people, the ideal prescription of reading glasses after cataract surgery is +2.00. However, for some people, they may desire a higher or lower power. In fact, simple calculations can determine the perfect prescription.
A +2.00 pair of reading glasses will work for most everyone. And these glasses can be picked up over the counter at any drug store or pretty much anywhere. But the perfect prescription depends on the person, the activity and even the type of artificial lens used during cataract surgery.
Why are reading glasses required after cataract surgery
Light that enters our eye is focused on our retina in the back of the eye. Two thirds of this light is focused by the cornea, the front clear part of our eye. The remaining third is focused by our natural lens inside our eye.
How light is focused in the eye, graphic by BruceBlaus, CC BY-SA 4.0, via Wikimedia Commons / modified from original
Over time, the lens becomes cloudy. When this cloudiness makes it hard to see, this lens is called a cataract - and cataract surgery is performed to remove this cloudy lens. (see also When Is A Cataract Ripe For Surgery?)
But the lens is responsible for a third of the focusing power of the eye; thus, it must be replaced with a new artificial lens.
When the surgeon selects this new artificial lens, the surgeon can choose how light is focused in the eye by selecting the power of the new artificial lens. This typically means picking a lens which will optimize the distance vision as much as possible. For many people who were near-sighted or far-sighted before cataract surgery, this means they will have much improved distance vision than what they were used to.
But these new artificial lenses aren’t flexible. They are called single focus or monofocal lenses. That means they can only allow you to focus at one point. If the lens is set at a distance focus, it means that things up close will be blurry. This requires reading glasses to see.
How is the reading glasses prescription calculated?
While the numbers on the reading glasses may not be entirely clear or make a lot of sense, the numbers actually have a meaning. In fact, the numbers are mathematically derived.
The power of any lens is measured in a unit called a diopter. With the diopter, one can calculate exactly where the lens is going to focus light. The focus of light (measured in meters) is the inverse of the diopter measurement.
Example:
We have a +3.00 diopter pair of reading glasses. This pair of reading glasses will focus things 1 / 3 = 0.33 meters in front of you. If we have a +1.00 diopter pair of reading glasses, things will be in focus 1 / 1 = 1 meter in front of you.
The higher or stronger the prescription, the more close up the reading glasses will focus.
The perfect distance
You may be thinking: Why not just get a strong pair of reading glasses so that you can see everything up close. And for some people this is the optimal solution. But it doesn’t work for everyone.
Intermediate working distance
Not all work happens within 0.33 meters away from us. In addition to up close and distance vision, there is a third category called intermediate vision. In fact, with the evolution of computers and the internet, more and more activities fall into this intermediate category.
If you correct vision to be very strong for up close, your computer vision remains blurry. You will most likely require a separate pair of computer glasses (a +1.50 is a good choice).
Some people don’t mind switching off between multiple pairs of reading glasses. Some keep a stronger pair (such as a +3.00) near their favorite book and keep a more milder computer pair near their computer. If this doesn’t sound like you, you may be better off with a +2.00 which covers both computer and reading adequately well.
Where you like to hold things to read
If you were near sighted before cataract surgery and are used to holding things real close to see, you may want to get a stronger pair of reading glasses. You will retain a lot of that muscle memory of bringing things in real close to see. A stronger pair of reading glasses can help mimic what your eyes used to be like.
The same goes if you are a shorter person or have shorter arms. You will naturally hold things closer than someone who is tall with long arms.
Measure your perfect distance
Want to be even more exact? You can measure the perfect distance yourself.
- Take out your tape measurer.
- Measure how far away you want to see.
- Take this distance in meters (if in centimeters, convert to meters first by dividing by 100).
- Divide that into 1 to get how many diopters in a reading glass you need.
As I’m typing this, I am sitting 0.74 meters away from the computer. The perfect pair of reading glasses for this distance would be 1 / 0.74 = 1.35 diopters.
Unfortunately, 1.35 diopter reading glasses don’t exist. So I would want to round up to +1.50 or down to +1.25 to cover that distance.
Summary
If you are going to buy a single pair of reading glasses, a +2.00 prescription will be the optimal way to sharpen up your reading and computer vision. But some people may desire a slightly higher or lower prescription. If in doubt, you can always try on the reading glasses in the drug store to get an idea of what would work best for you.
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