Negative "cyl" (cylinder) in an eyeglass prescription indicates the amount of lens power needed to correct for nearsighted astigmatism.
Here's a more detailed explanation:
Astigmatism occurs when the cornea or lens of the eye is not perfectly round, causing blurry vision at all distances. The "cylinder" (cyl) value in your prescription addresses this irregularity.
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Cylinder Power: This number represents the lens power needed to correct astigmatism. It's always accompanied by an "axis" value, which specifies the orientation of the astigmatism.
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Negative Sign (-): A negative sign before the cylinder value signifies that the lens is correcting for nearsighted astigmatism (myopic astigmatism). This means the eye focuses light in front of the retina in one meridian, which the cylindrical lens corrects to focus properly on the retina.
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Example: If your prescription reads CYL -1.00, it means you have 1.00 diopters of cylinder power to correct nearsighted astigmatism.
In contrast, a positive cylinder value (+) indicates farsighted astigmatism (hyperopic astigmatism).
In summary, a negative cylinder ("- cyl") value in your eyeglass prescription is a measure of the lens power needed to correct nearsighted astigmatism, helping you see clearly by compensating for the irregular curvature of your eye's cornea or lens.
