Accommodative response was measured on fifteen subjects as they read successive lines on a standard Bailey-Lovie acuity chart at varying levels of defocus.
Letter size had a measurable effect on the accuracy of the accommodative response. At large letter sizes the average accommodative response at a 4 D demand was 2.97 ± 0.36D. At smaller letter sizes the average response was significantly larger at 3.44 ± 0.24D (F3.2,45.3 = 22.4, p<0.0001, repeated measures ANOVA). The first significant increase in accommodative response was noted at a letter size of 9.7 minutes of arc (3.27 ± 0.22D; t14= 5.0, p=0.014).
The relationship between logMAR acuity and myopic defocus was linear between 0 and –3D. The results were similar under cycloplegia; myopic and hyperopic defocus had roughly the same effect on visual acuity. Taking into consideration all of these factors; excessive lag during reading might be in the range of 1.75 to 2.25D.