Added this new study:
The French DESIR study of people diagnosed with pre-diabetes confirmed what the studies cited above have found. Subjects who developed retinopathy over a 9 year period had an average fasting blood sugar of 130 mg/dl and an average A1c of 6.4%. Those who did not develop retinopathy over this period had an average fasting blood sugar of 108 mg/dl and an average A1c of 5.7%.
Hemoglobin A1c and Fasting Plasma Glucose Levels as Predictors of Retinopathy at 10 Years: The French DESIR Study. Massin P. et al. Arch Ophthalmol.2011 Feb;129(2):188-195.