Definition

Noun: - Aperture ratio; f-number: The focal ratio is a dimensionless number that expresses the ratio of a lens system's focal length to the diameter of its entrance pupil (the effective aperture). It is a key parameter in optics and photography, determining the light-gathering ability and depth of field of a lens.

Usage

The term "focal ratio" is used primarily in technical contexts such as optics, astronomy (for telescopes), and photography. It is often denoted as f/N, where N is the numerical value of the ratio (e.g., f/2.8, f/16). A lower focal ratio (e.g., f/1.4) indicates a "faster" lens that admits more light, while a higher focal ratio (e.g., f/22) indicates a "slower" lens that admits less light.

Examples

• Noun:
  • A telescope with a focal ratio of f/10 is well-suited for planetary observation.
  • For astrophotography, a lens with a low focal ratio, like f/2.8, is preferred to capture more light from dim stars.
  • The photographer adjusted the camera's focal ratio to f/8 to achieve a greater depth of field for the landscape shot.

Advanced Usage

• "Fast" vs. "Slow" lens: A lens with a small focal ratio number (e.g., f/1.8) is called a "fast" lens because it allows for shorter exposure times. A lens with a large focal ratio number (e.g., f/16) is called a "slow" lens.
  • The f/1.4 prime lens is excellent for low-light photography due to its fast focal ratio.

Variants and Related Words

• f-number (n): A synonym for focal ratio, expressed as f/N.
  • The f-number is a critical setting for controlling exposure.
• Aperture (n): The opening in a lens through which light passes. The focal ratio is calculated from the aperture's diameter.
  • A wider aperture corresponds to a lower focal ratio.
• Relative aperture (n): Another technical term synonymous with focal ratio.

Synonyms

• f-number
• Relative aperture
• Aperture ratio
• F-stop (Note: While closely related, "f-stop" technically refers to the specific increments on a lens's aperture scale, each representing a different focal ratio.)

Related Phrases

• Stopping down: The act of increasing the focal ratio number (e.g., moving from f/4 to f/8) to reduce the size of the aperture.
  • Stopping down to a higher focal ratio increases the depth of field.
• Wide open: Using the lens at its smallest possible focal ratio number (largest aperture).
  • Shooting wide open at f/1.8 creates a very shallow focus.

Noun

1. the ratio of the focal length to the diameter of a (camera) lens system