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

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.

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

Not the answer you're looking for? Browse other questions tagged or ask your own question.