Tolerance doses & manifestations for normal eye tissues:

Retina 45 - 65 Gy - blindness
Lens 10 - 18 Gy - cataract
Optic nerve 50 - 65 Gy - blindness

from table 21.9, pg. 504 in:
Treatment Planning in Radiation Oncology
Faiz M. Khan & Roger A. Potish Eds.
Wiliams & Wilkins Pub. 1998