F
From The Glossary of Digital Photography
Refers to the f-number or f-stop, as in f /2.8. The f is an abbreviation for factor, focal, and focal length. See f-stop

ONLINE GLOSSARY OF DIGITAL PHOTOGRAPY
Refers to the f-number or f-stop, as in f /2.8. The f is an abbreviation for factor, focal, and focal length. See f-stop