This population-based cohort study assessed the association of refractive error with the incidence and progression of diabetic retinopathy (DR); 1562 eyes of 860 patients with diabetes and gradable retinal photographs were included. Baseline data was taken between 2004-2009 and follow-up between 2011-2015. Spherical equivalent, axial length, anterior chamber depth and corneal curvature were assessed. Diabetic retinopathy progression was defined as an increase in severity at follow-up. Out of the 1562 eyes 20 were excluded as they had vision threatening DR (VTDR) at initial review; 1273 eyes had no DR at initial review and 269 eyes had non-vision threatening DR. At follow-up 164 of 1273 eyes had incident DR, 17 of 1542 (1.1%) eyes had incident vision-threatening DR and 75 of 269 (27.9%) eyes with baseline DR experienced progression. A longer axial length (per millimetre increase) (risk ratio 0.58; 95% confidence interval 0.38-0.88) was associated with a lower risk of incident DR, independent of spherical equivalent, anterior chamber depth and corneal curvature. No other associations were found. In addition there was no significant association between axial length and incident VTDR or DR progression however this may be due to small numbers in this study. These results support other work done by the same group and by others however further research is needed to understand the mechanisms involved.