Why we add half of cylinder to spherical when reducing cylinder

After staring at a lot of charts about my eye measurements, I think I’ve finally internalized some things about how to think about cylinder and what it actually means when you get rid of it in your prescription. The rule that says you can reduce your cylinder by adding half of it to your spherical is what I’m talking about.

The spherical and cylinder values are the two extremes of lens power your eye needs. It’s a way to represent the messy complexity of your eye’s deficiencies with just two numbers (plus the axis).

When they go to create your glasses, the cylinder value is added to your spherical and represents the higher power of the lens. So, for example, if your spherical is -2.00 and cylinder is -1.00 then your lens has a range of powers in it from -2.00 to -3.00.

If we want to simplify the numbers for your eyes even farther, down to a single number, we can calculate the “spherical equivalent” for it. Half way in-between the two extremes is the spherical equivalent. In the previous example, the spherical equivalent would be -2.50. It just so happens, that if you totally get rid of your cylinder using the “half rule” and move it all into spherical, you are calculating the spherical equivalent for your eye. And that makes sense since you are having to pick a single number to represent the power that your entire eye needs. The compromise you are making for having one value for your whole eye is that the more myopic part of your eye, that needed the cylinder, is getting a less powerful lens and the less myopic part of your eye that needed the spherical is getting a more powerful lens.

I think in reality, this compromise means you can expect:

  • Because of the stronger spherical, you’ll have less blur in the eye and your distance-to-blur may increase.
  • But because you are replacing the stronger cylinder with the weaker spherical, the double vision from your astigmatism might increase.
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