Fig. 5. Comparison of fixed-area and fixed-force applanation tonometers. In fixed-area applanation, a constant area of applanation a is used for every measurement. i) A greater force of applanation (F) is required to applanate this area in an eye with higher intraocular pressure (P). ii) A smaller force (f) is required to applanate the same area in an eye with lower intraocular pressure (p). In fixed-force applanation, a constant applanating force (F, the weight of the tonometer) is used for every measurement. iii) A smaller area of applanation (a) is achieved with this force in an eye with higher intraocular pressure (P). iv) A larger area of applanation (A) is obtained with the same force in an eye with lower intraocular pressure (p).