Sky science
Focal Ratio
Direct answer
Focal Ratio is telescope focal length divided by clear aperture, written as an f-number.
It relates focal length to aperture.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Telescope Aperture, Telescope Focal Length, Magnification, Exit Pupil
It relates focal length to aperture.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Focal Ratio
Photography popularized f-number language across camera and telescope optics.
A one-minute story
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How the moving planets were watched, named and found, and what a transit is.
Why Focal Ratio matters
Focal Ratio influences image scale, field design, aberration demands, and extended-source exposure per detector area.
How to read Focal Ratio
Do not use focal ratio alone to compare total light collected from point sources.
Focal Ratio in practice: an example
A 1000-millimeter focal length with 200-millimeter aperture is f/5.
Where readings of Focal Ratio go wrong
Visual brightness also depends on eyepiece exit pupil and target type.
Sources and editorial method
Research references used for this entry include:
- European Southern Observatory: Technology for Telescopes - Observatory reference for aperture, angular resolution, atmospheric seeing, mirrors, and telescope technology.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
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Common questions
What is focal ratio?
Focal Ratio is telescope focal length divided by clear aperture, written as an f-number.
What do people get wrong about focal ratio?
Visual brightness also depends on eyepiece exit pupil and target type.
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