focal length
US /ˈfoʊ.kəl ˌleŋθ/
UK /ˈfoʊ.kəl ˌleŋθ/
1.
the distance from the center of a lens or curved mirror to its focal point
:
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Adjusting the focal length changes the magnification of the image.
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A longer focal length results in a narrower field of view.