To over or undercorrect, or to target zero power outcome depends on a few things:
1) Age of patient: tendency is for younger patients to target overcorrection. 0.25D to 0.5D overcorrection is common for those below 30 year old. One of the reasons is to "reserve" for future regression.
2) Consistency of refraction results: sometimes the power measurements do not completely agree, resulting in discrepancy between glasses or contact lenses worn and clinic refraction results. Refraction results may also differ with and without relaxing eyedrops.
In these cases, the surgeon may choose to follow the stronger prescription, to keep distance vision clear.
3) For patients close to 40 year old, tendency is to undercorrect in at least one eye to preserve near vision for a bit longer and delay onset of presbyopia symptoms.
Hope this clears your doubt. Cheerio!
Dr Daphne Han
