Ibn al-Musayyib (may Allah have mercy on him) said that a man was struck in the eye and his sight was weakened. The case was brought to Sayyiduna Ali (may Allah be pleased with him). He bound up his healthy eye and ordered a man to walk carrying an egg (until the point) where his sight came to an end. Then the other eye was checked in the same way, and the two were almost equal. He was given diyah in proportion to the sight he was found to have lost.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16235