Probe frequency defines the wavelength of the system, which in turn sets the minimum distance between two adjacent structures that the system can resolve. For example, a 20 MHZ pachymeter probe frequency has a wavelength of

L = 82 microns

This is derived by dividing the velocity of sound in the cornea (1640 m/s), by the probe frequency (20MHz). The minimum distant that a 20MHz system can resolve is given by L/2. This sets the minimum measurable distance (between two adjacent structures) a 20MHz system can measure at 41 microns. In comparison, the minimum distance for a 50 MHz system is 16.4 microns.

This discussion shows that the probe frequency is not really a source of measurement error or accuracy, but rather it defines the minimum resolvable distance. As it turns out, most measurable structures of interest within the cornea are greater than 41 microns in thickness and therefore a 20MHz probe frequency is very adequate for making these measurements.