Correction to: Eye (2011) 25, 1333–1336; doi:10.1038/eye.2011.169; published online 22 July 2011

Since the publication of the above article, the authors have noticed that further testing of the spreadsheet calculations in the article has revealed, in a predictable and limited set of special circumstances, that the spreadsheet calculations return a recognisable error. No candidates have been incorrectly scored as a result. The error is rectified in the following code, which also simplifies the number of calculation steps required.

If, at this stage, a dioptric power of the error is desired, as previously described, this correcting refraction can be converted as follows:

Unconventional results still occur with special scenarios, such as a one axis being entered as zero and the other as 180 (results in zero cylinder power at 135), and one axis 10, the other (minus) −170 (result zero cylinder power at 90). The calculated dioptric defocus equivalent would still be calculated correctly.

The authors would like to apologise for this mistake.