Accommodation limits induced optical defocus in defocus experiments.

A common method to induce optical defocus is by means of positive lenses. However, accommodation, pupil size, and depth of focus are important factors determining the amount of defocus achieved. We therefore studied the effect of accommodation on visual acuity with a natural pupil (miosis) and in cycloplegic mydriasis. Additionally, we looked at the amount of accommodation and pupil size fluctuation during the recording of a mfVEP. We found that the amount of defocus induced with lenses of positive power is limited in non-mydriatic experiments by accommodation and actual pupil size and may show marked variation during mfVEP recording. We therefore recommend using a mydriatic drug and, if required, an artificial pupil for defocus experiments.