The distance between the optical centre of a lens or pole of a spherical mirror and its principal focus.
The distance between a lens or mirror and its focal point. Together with the aperture, the focal length is one of the main properties of a lens or mirror. The longer the focal length, the larger the image scale.
The distance from a lens to the focus point where parallel rays of light hitting the lens are brought together. The focal length of a lens determines its magnification and its field of view, with a shorter length meaning lower magnification but a larger field of view.
Focal length
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- Prospero
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- prostaglandin
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- protactinium/thorium isotopes
- Protagoras of Abdera (c.490–420)
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- protectionism