'Diagnosed with diabetes after the New York Marathon'
The proportion of diabetics who go blind or suffer sight loss has almost halved since a new national retinopathy screening programme started in 2007.
Swansea University research over eight years has now been published in the British Medical Journal.
New certifications for severe sight impairment have fallen from 31.3 to 15.8 per 100,000 people.
Diabetics aged over 12 are offered annual screening and health experts say the study shows a "clear benefit". But still 20% of diabetics do not take up the offer.
Robert Lee, now 65, from Cardiff has been having annual checks since he was diagnosed with diabetes in 2004.