The Amsler grid is commonly used for self-monitoring, but is qualitative and has low sensitivity. The M-chart reduces the amount of spatial redundancy (‘filling in’) in the Amsler grid. The Orientation Discrimination (OD) test studied in this paper consists of a group of short lines that are displayed only briefly, thus minimising the influence of ‘filling-in’ and ‘averaging out’ that can happen in Amsler grid or M-chart. Forty-eight patients with idiopathic epiretinal membranes (ERM) were included in this observational cross-sectional study, and healthy age-matched individuals were assigned to a control group. All participants were subjected to Amsler grid, optical coherence tomography (OCT) and OD test (where OD threshold, orientation discrimination threshold (ODT), was measured). ODTs were significantly higher in patients with ERM (both fovea-sparing and fovea-involving) compared to the control group. Of these, the Amsler grid only yielded positive findings in a quarter of subjects. ODT also correlated with internal retinal layer thickness on OCT, however, it was noted that disturbance produced by ERM was not wholly captured by retinal thickness measures. Future studies should pay more attention to changes in microstructure in individual retinal layers and how these correlated with metamorphopsia. The authors conclude that OD test can evaluate functional changes that are too subtle to be detected by the Amsler grid.