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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)

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  • \$\begingroup\$ Mainly because the focal length of any lens is generally measured when focused on infinity. \$\endgroup\$
    – Michael C
    Mar 13, 2015 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.

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  • \$\begingroup\$ But for a simple lens, these are the same thing, so just saying this doesn't quite answer the question. \$\endgroup\$
    – mattdm
    Mar 9, 2015 at 19:36

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