Sir,

Thank you for your observation that the Royal College of Ophthalmologists (RCOphth) guidelines on IOL formulae quoted in the National Biometry Audit II paper1 are not the same as the current RCOphth guidelines.2 This is because the guidelines changed in the interval between acceptance of this paper and its publication. While all modern formulae achieve good results in the common axial length range (when A constants are customised separately for IOL Master or ultrasound biometry), it is of interest to note, for example, that an eye with an axial length of 21.00 mm and K values of 7.80 mm, the Hoffer Q and SRK/T formulae recommend IOL powers of 32D and 31D, respectively, to achieve emmetropia, which suggests that for short eyes different formulae are not likely to be equally accurate. Large data sets of short or long eyes are needed to prove the superiority of one or other formula for eyes at the extremes of the axial length range and the Cataract National Dataset in the UK will be one way of achieving such data sets.