How many letters does the top answer for “relating to or using sight” have?

The most common answer is optic (5 letters).

Crossword clue: relating to or using sight

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

optic 5 letters wn-def

the organ of sight

visual 6 letters wn-def

relating to or using sight

of or relating to or using logarithms 50% similar → logarithmic