Blindness exists for purposes of establishing eligibility for AB when the central
visual acuity is 20/200 or less in the better eye with correcting glasses.
Blindness also exists when the central visual acuity is more than 20/200 in the better
eye with correcting glasses, but when by a rough test, a marked field defect is shown.
A marked field defect is one in which the peripheral vision has contracted to such
an extent that the widest diameter of the visual field subtends at an angular distance
of no greater than twenty degrees.