1. The ratio of the focal length to the diameter of a (camera) lens system
   - focal ratio, f number, speed

Derived forms: stop numbers

Type of: ratio

• genus Philaenus
• genus Phillyrea
• genus Philodendron
• genus Philohela
• genus Philomachus
• genus Philophylla
• genus Phlebodium
• genus Phlebotomus
• genus Phleum
• genus Phlomis
• genus Phlox
• genus Phoca
• genus Phocoena
• genus Phoenicophorium
• genus Phoeniculus
• genus Phoenicurus
• genus Phoenix
• genus Pholas
• genus Pholidota
• genus Pholiota
• genus Pholis
• genus Pholistoma
• genus Phoradendron
• genus Photinia
• genus Photoblepharon