Cylinder

Cylinder lenses correct for the most common kind of astigmatism. It's represented as CYL on prescriptions. The lens is shaped like a cylinder rather than a sphere, bending light in one direction, but not in the other. The direction is specified by the Axis.

Positive vs. Negative Cylinder

Eyecare professionals have individual preferences for writing out positive vs negative values for cyl. For discussion with other EM participants, it is usually recommended to convert positive cylinder values into their negative equivalent, as the negative cylinder values are potentially more intuitive from a reduction point of view.

• For example, there are participants who get surprised when they remove plus cylinder, and then find that the glasses they order are too strong.
• There are tools that will convert between equivalent prescriptions of plus or negative cylinder
• Gemily's video explaining plus vs negative cylinder

See also

• Astigmatism