Lens Form & Analysis
Cylinder Axis

A cylindrical shape is flat in one direction and curved in the other. As we now know, it is the curve of a lens that gives it its power. Therefore the direction of the cylinder with no curve also contains no power and is called the cylinder axis.

Figure C above shows a cylinder positioned on a protractor where the axis of the cylinder is positioned along the 30 degree meridian. If the power of the cylinder is +2.00D, we could say this was a plano sphere (since there is no spherical component here, only a cylinder) which is combined with a +2.00 diopter cylinder in which the axis is positioned along the 30 degree meridian. Since that's a lot to say, we use a shorthand notation to indicate the same thing: Pl + 2.00 x 30.

In a pair of spectacles, the orientation of the cylinder axis is determined as if a protractor were superimposed over each lens as indicated in figure D above.