Corneal astigmatism: Cornel astigmatism above 4 diopters can cause error in readings due to distortion of the mires. To compensate, twist the probe so that the red line on it corresponds to the minus cylinder axis.
Corneal thickness: Central corneal thickness will affect your tonometry readings if it is significantly more or less than the average thickness of 555 microns. To adjust your readings, for every 25 microns add or subtract 1mmHg. Add if the thickness is lower than normal, subtract if it is higher than normal.
