OpticBench

Guide

Reading a lensometer

A lensometer shows an astigmatic lens as two sets of lines that come clear at two different drum readings. Those two readings are the two principal meridians, and the written Rx falls straight out of them.

What the instrument presents

A sphere brings the whole target clear at one drum reading. A sphero-cylinder splits the target: the narrow sphere lines come clear at one power, the wide cylinder lines at another, 90 degrees apart. The gap between the two drum readings is the cylinder power; the axis wheel orientation when the sphere lines are clear is the cylinder axis.

The minus-cylinder routine

  1. Center the lens, target roughly focused, axis wheel swept until the sphere lines are unbroken.
  2. Turn the drum to the most plus position where the sphere lines are sharp and stop there first. That drum reading is the sphere.
  3. Read the axis wheel: that is the cylinder axis.
  4. Turn the drum toward minus until the cylinder lines sharpen. The difference between the two readings, as a minus value, is the cylinder.
  5. Example: first reading -2.00, second reading -3.50, axis 180. The Rx is -2.00 -1.50 x 180.

Stopping at the more minus reading first yields the plus-cylinder form of the same lens; transposition converts between the two on paper.

The dial, live

The meridian power page draws the dial for any Rx: the cylinder axis, an inspection meridian you rotate, and the power in that meridian computed from F = S + C x sin^2(theta - axis). Watching the readout swing between the two principal powers as the meridian sweeps is the fastest way to internalize what the drum is doing.

The optical center and prism

With the target centered on the reticle the lens is at its optical center; dot it there. A target displaced from the center shows prism, one ring per prism diopter on most reticles, which is Prentice's rule made visible. Verified powers then go against the order with the tolerance checker.

Frequently asked questions

What do the two line focuses on a lensometer mean?

They are the two principal meridians of the lens. The power drum reading at the first clear sphere line is the sphere; the drum difference to the second line is the cylinder; the axis wheel at the first line gives the axis.

Why does my lensometer read minus cylinder?

Convention. Stopping at the less plus (more minus) drum reading first yields minus-cylinder form. Stopping at the more plus reading first yields plus-cylinder form; the lens is the same either way.