Cylinder

Cylinder is a term used in optometry and ophthalmology to refer to the power and orientation of astigmatism in an eye or in a corrective lens.

Definition

A cylinder in optometry is a lens power that is not spherical. It has different powers in different meridians. This is in contrast to a spherical power, which has the same power in all meridians. The power of a cylinder lens can be positive or negative.

Astigmatism

Astigmatism is a common vision condition that causes blurred vision. It occurs when the cornea (the clear front cover of the eye) is irregularly shaped or sometimes because of the curvature of the lens inside the eye. An irregularly shaped cornea or lens prevents light from focusing properly on the retina, the light-sensitive surface at the back of the eye. As a result, vision becomes blurred at any distance.

Cylinder in Prescription

In an eyeglass prescription, the cylinder number indicates the amount of lens power for astigmatism. If nothing appears in this column, either you have no astigmatism, or your astigmatism is so slight that it is not really necessary to correct it with your eyeglass lenses.

Cylinder Orientation

The orientation of the cylinder is described by the axis. The axis is the lens meridian that is 90 degrees away from the meridian that contains the cylinder power. The axis is defined with a number from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian.

See Also

• Optometry
• Ophthalmology
• Astigmatism
• Lens (optics)

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