Visual acuity degradation under positive lens–induced myopic defocus at nominal chart contrast levels in young adults: a repeated-measures study

OBJECTIVE: To quantify visual acuity (VA) degradation across nominal chart contrast levels under graded positive lens-induced myopic defocus and to determine whether corrected low myopes differ from emmetropes in their acute response to optical blur. METHODS AND ANALYSIS: In this repeated-measures study, 300 adults aged 18-35 years (150 low myopes and 150 emmetropes) underwent monocular logMAR VA assessment at 6 m. Positive spherical lenses (+0.50 to +2.00 D) were added over best correction to induce myopic defocus. VA was measured at five nominal chart contrast levels (100%, 70%, 50%, 25% and 13%) under natural pupil conditions. A repeated-measures general linear model evaluated the effects of defocus and contrast on VA, with refractive group and gender as between-subject factors and age as a covariate. RESULTS: Defocus produced a significant decline in VA (p<0.001, partial η²=0.471), with progressive degradation as defocus increased. Progressively lower nominal chart contrast levels also significantly worsened VA (p<0.001, partial η²=0.237). A small but significant Defocus×Contrast interaction was observed (p<0.001, partial η²=0.015), indicating slightly greater blur-related degradation at lower nominal contrasts. No clinically meaningful interactions involving refractive group were observed. Although myopes demonstrated slightly poorer overall VA than emmetropes (mean difference 0.041 logMAR; p<0.001, partial η²=0.056), refractive status had little influence on the overall pattern of acuity decline across defocus or contrast levels. CONCLUSIONS: VA degradation under positive lens-induced myopic defocus is primarily determined by the magnitude of optical blur, with reduced contrast exerting an additional but smaller effect. Corrected low myopes and emmetropes show broadly comparable functional responses to acute defocus under natural viewing conditions, although subtle subgroup differences cannot be excluded.