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Cylinder power is a measurement on an eyeglass prescription that shows the amount of lens power needed to correct astigmatism. If your prescription includes a number for cylinder power, it means your eye is not perfectly round. If this part of your prescription is blank, you do not have astigmatism that needs correction.
read more about cylinder power ...An eye with astigmatism is shaped more like a football than a round basketball. This causes light to focus on two different points instead of one, which makes vision blurry. The cylinder power is an extra power added to the lens in just one specific direction. Its job is to correct for the football shape and bring all the light to a single, sharp focus point.
On your prescription, the cylinder is usually the second number and is labeled "CYL." It will have a minus (-) or plus (+) sign in front of it. The further the number is from zero, the more astigmatism you have. The cylinder power always works with an axis number, which tells the lab where to place the correction.
Sphere power, labeled "SPH," corrects for nearsightedness or farsightedness and is the same in all directions of the lens. Cylinder power is different because it only provides correction in one specific direction. You can think of sphere power as the overall focus for the eye, while cylinder power is a fine-tuning adjustment for the astigmatism.
To understand cylinder power, imagine a clock face on your eye. If you have astigmatism, your eye has two "principal meridians" that are 90 degrees apart. For example, one might be from 12 o'clock to 6 o'clock, and the other from 9 o'clock to 3 o'clock. The cylinder power represents the difference in curvature between these two meridians. It effectively adds power to only one of these lines to make it match the other, turning the football shape back into a basketball shape optically.
A high cylinder number simply means you have a higher amount of astigmatism. It is not "bad," but it does mean that a precise correction is needed to give you the clearest and most comfortable vision.
In the United States, most eye doctors write prescriptions using a minus cylinder form. This is just a standard notation. The same prescription can also be written in a plus cylinder form.
No. Many people have no astigmatism or a very tiny amount that does not affect their vision. For these people, the cylinder part of the prescription is left empty.
Your eye doctor measures your cylinder power and axis during your eye exam. If you get new glasses and feel that things look tilted or your vision is not as sharp as it should be, you should let your doctor know. It could be a sign that your cylinder power or axis needs to be adjusted.
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