The details of refractive and visual acuity measurement have been examined using the International Standards Organization (ISO) guidelines for the estimates of uncertainty. It is clear that there is a number of sources of uncertainty in quoted values. Some of these can be analysed statistically, using the ISO guidelines and can be used to estimate an uncertainty in the final recorded values. These should be regarded as minimum uncertainties because there are other factors involved such as unwanted accommodation, which will affect the final outcome. The analysis shows that the standard uncertainty (+/- one standard deviation) is about 0.3 D in refractive error measurement and about 0.04 in LogMAR acuity. The normally quoted expanded uncertainty, which provides a 95 per cent confidence level, would then be 0.6 D in refractive measurement and 0.08 in LogMAR acuity.