This technique measures the curvature of the central cornea over 3 - 6 mm. It is used to assess the curvature, power, and toricity of the cornea, as well as the presence of any corneal distortion. The two types of keratometer are one-position and two-position. They differ in the way that the system of ‘doubling’ is used to measure the separation of mire images. Doubling helps to counteract rapid eye movements that would otherwise make taking the measurement very difficult. One-position keratometers use variable doubling and their mires have a fixed separation. The separation of the mires is found by varying the doubling power.