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If the definition of prime lens says that their focal length is fixed, then how come you can still alter the focal length when focusing? (either MF or AF)

marked as duplicate by mattdm, Philip Kendall, Caleb, MikeW, chuqui Mar 9 '15 at 19:11

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  • Mainly because the focal length of any lens is generally measured when focused on infinity. – Michael C Mar 13 '15 at 7:41

Focusing doesn't require changing the focal length of a lens, it changes the distance of (the center of) the lens to the sensor or film.

  • But for a simple lens, these are the same thing, so just saying this doesn't quite answer the question. – mattdm Mar 9 '15 at 19:36

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