Calculating the error in refractive error.

Aims And Purpose: To demonstrate a quick way of calculating the optical difference between two refractions using vector analysis, and to express this as a score for examination purposes.

Methods: An existing formula is applied, converted to a defocus equivalent, and then converted to a score. The formula is set out in an Excel spreadsheet.

Results: The spreadsheet enabled rapid assessment of the difference between two refractive errors. Examples are demonstrated.

Conclusion: The spreadsheet was successful in enabling a comparison of any two refractions, expressing the difference either as a correcting (third) refraction, a defocus equivalent or a one-figure score.