Definition

Noun: - The distance between the optical center of a lens (or the surface of a mirror) and its focal point, where light rays converge to form a sharp image. It is a fundamental property that determines the lens's magnifying power and angle of view.

Usage

• Focal length is a key specification for camera lenses, microscopes, telescopes, and other optical instruments.
• It is typically measured in millimeters (mm).
• A shorter focal length provides a wider field of view, while a longer focal length provides a narrower field of view and greater magnification.

Examples

• Noun:
  • A 50mm focal length is considered standard for a full-frame camera.
  • The telescope's long focal length allows for detailed observation of distant planets.
  • To capture the entire landscape, use a lens with a short focal length.

Advanced Usage

• "Effective focal length": The focal length adjusted for a specific sensor size or optical system, which affects the actual field of view.
  • On a camera with a smaller sensor, a 35mm lens has a longer effective focal length.

Variants and Related Words

• Focal point (n): The point where light rays converge after passing through a lens or reflecting from a mirror.
• Focal plane (n): The plane, perpendicular to the optical axis, where the image is in sharp focus.

Synonyms

• Focal distance: A less common but technically equivalent term.

Related Phrases

• Fixed focal length: Describes a prime lens with a single, non-adjustable focal length.
  • Photographers often prefer fixed focal length lenses for their sharpness.
• Focal length multiplier: A factor (like 1.5x or 1.6x) used to calculate the effective focal length when using a lens on a camera with a sensor smaller than a full-frame.
  • With a 1.5x crop factor, a 100mm lens has an effective focal length of 150mm.

Noun

1. the distance from a lens to its focus