Effect of defocus on visual field measurement.

Defocus increased visual field contrast thresholds for stimuli equal to or smaller than Goldmann perimeter target III (26 min arc diameter). Beyond 30 degrees-40 degrees from the centre of the visual field, defocus had little effect on contrast thresholds. For accurate clinical perimetry, ametropia and presbyopia should be corrected for small targets (less than or equal to target III) within 30 degrees-40 degrees of fixation with either a spectacle lens (if required power less than +/- 10 D) or a contact lens. Larger stimuli are little affected by defocus. Outside this central 30 degrees-40 degrees field, correction with contact lenses should be provided if fixation is poor due to ametropia. Scotomata for which no cause is evident should be further investigated to find whether they have a refractive basis.