Ever since the recognition that glaucoma was associated in many patients with a firm eye, ophthalmologists have been attempting to measure intraocular pressure (IOP) clinically. Prior to the introduction of Goldmann Applanation Tonometry (GAT) in the 1950s, tonometry techniques were inconvenient and unreliable. Professor Goldmann’s tonometer rapidly gained widespread acceptance following its introduction – it was reasonably-priced, based on easily-understood physical principles, fitted seamlessly into the workflow of the slit-lamp exam, and appeared to provide accurate, reproducible measurements. GAT’s status as a tonometry “Gold standard” went largely unchallenged for 50 years, even though Professor Goldmann himself drew attention to various potential sources of error for the device in his first description of his tonometer [1]. In particular, Goldmann and Schmidt acknowledged that their design assumptions were based on a central corneal thickness (CCT) of 0.5 mm (500 µm) and that the accuracy of their device would vary if CCT deviated from this value – “Under conditions which differ considerably from our measurement conditions (abnormally thick or thin cornea, for example…), errors of several millimeters are to be expected” [1]. Given the paucity of published data at the time, 500 µm seemed a reasonable assumption for the “average” patient. We now know CCT varies greatly among the general population, to a degree that impacts the accuracy of GAT in daily practice.