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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)

  • Mainly because the focal length of any lens is generally measured when focused on infinity. – Michael C Mar 13 '15 at 7:41
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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. – Please Read Profile Mar 9 '15 at 19:36

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