How big of a visual field defect would be an eye problem while driving?

20-30% reduction. Usually after a 20-30% reduction, difficulty with eye fatigue and loss of effective night vision is noted. Believed that excessive hooding to that level forces drivers to lift their head when driving at night, resulting in excessive glare and loss of "gray" colors needed for depth perception. Use of visual field analysis in most eye doctor offices can measure the degree of loss.