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

• arcella
• Arcellidae
• Arceuthobium
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• arch
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• archaean
• archaebacteria
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• archaeo-
• archaeobacteria
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• Archaeopteryx lithographica
• archaeornis
• Archaeornithes
• archaeozoic
• Archaeozoic aeon
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