Purpose <p>Visual acuity (VA) assessment is crucial for evaluating visual function, but inter-clinic VA measurement variations were observed in patients with epiretinal membrane (ERM). Inter-clinic variations in acuity chart luminance exist because the International Organization for Standardization (ISO) recommends a range of 80–320 cd/m² for VA charts. We investigated the effects of luminance within this ISO range on the VA in patients with ERM.</p> Study design <p>Prospective observational study.</p> Methods <p>We analyzed data from 187 patients with ERM and a normal fellow eye with a decimal VA of ≥1.0 at 320 cd/m² OU. Baseline VA was measured at 320 cd/m², then reduced in 0.1-log unit steps using neutral density filters. Metamorphopsia was evaluated with the Amsler Grid Test. The luminance causing a two-line VA reduction from 320 cd/m² was measured before and after ERM removal.</p> Results <p>In normal fellow eyes, a two-line VA reduction occurred at approximately 40 cd/m², indicating stable VA within the normal ISO range, while in affected eyes, it happened at around 150 cd/m². The luminance difference causing a two-line VA reduction between eyes with and without metamorphopsia was not significant. For surgical patients, the luminance causing a two-line VA reduction decreased over time: from 153 cd/m² preoperatively to 36 cd/m² at 6 months postoperatively.</p> Conclusions <p>Considering the luminance is critical when measuring VA in patients with ERM.</p>