focal ratio
1
7.8
- noun
- /ˈfoʊkəl ˈreɪʃioʊ/
- Specialized
:
relación focal
- aperture and focal ratio
- focal ratio of f/6
- telescope with a focal ratio
Examples
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For observations, we use the 65-cm telescope with a focal ratio of 1/3.6 and a CCD camera.
Academic text (2000) -
The telescope has a focal ratio of focal ratio f/5, which allows for brighter images during stargazing.
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A lower focal ratio means that the lens can gather more light, resulting in better image quality.
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To achieve optimal clarity, the camera requires a focal ratio of at least f/2.8 when shooting in low light.
Synonyms
72
8.5
How much light a camera lens lets in
A value that shows how wide a camera lens opens to let light in
A number that shows how wide a camera lens opens to let light in
Surface Forms
Morphology
The components 'focal' (relating to focus/focal length) and 'ratio' (a numeric relation) give a learner the general idea that this is a numeric relationship connected to the optical focus, so the broad concept is inferable. However, the precise technical sense — that it specifically denotes focal length divided by aperture diameter and relates to image brightness/speed — is specialized and not fully predictable from the parts for a B1 learner without domain knowledge.
Etymology
The term focal ratio comes from comparing the lens focal length (the distance light travels to make an image) to its diameter (the size of the front opening). Imagine a long, thin tube and a short, wide one: the short, wide tube gives a brighter image, so the ratio tells you how 'fast' or 'bright' the lens will be.