Optical biometry using the Zeiss IOL Master has become synonymous with measurement precision, simple operation and excellent outcomes. The IOL Master is easy to use, however it is important to understand the procedure and technique and that the measurements are applied to modern formulae using optimized A constants. Following on from ‘Accurate optical biometry using the IOL Master’ ( Millbank, 2011 ), the aim of this article is to discuss and explain the formulae to which the biometry measurements on the IOL Master are applied and to attempt to demystify the commonly misunderstood lens constant' issue.