TT #130 · Parshas Tazria · Aliyah 2 · Passuk 12
English: If the eruption spreads out in the skin, so that the eruption covers all the skin of the affected person from head to foot, as far as the priest can see,
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:12
Torah Temimah cites Sifra (Toras Kohanim) for excluding a kohen with impaired vision in either eye from examining lesions.
Sifra — Vayikra 13, Midrash Sefer
English: A priest blind in one eye, or whose eyesight has dimmed, may not inspect lesions, as the verse says, “for every appearance of the priest’s eyes.”
Textual derivation
The rule also excludes a kohen with partial or diminished vision, not only one who is completely blind.