0

If I have a 200mm lens, and want to focus at, say, 450mm from the camera, two questions:

  1. can I (do I need to) use bellows to achieve this?
  2. is there a formula for how to work out how much extension I need on the bellows to achieve the minimum focal length?
MikeW
  • 33,408
  • 10
  • 88
  • 123
James
  • 215
  • 1
  • 6
  • It's hard to answer this because you are completely confused. A 200mm lens has to do with its focal-length and determines the angle-of-view when mounted on a certain sensor-size. This has nothing to do with focus-distance. – Itai Apr 22 '16 at 03:40
  • I understand that. I guess I naively thought that the minimum focal distance would be a function of the lens length. – James Apr 22 '16 at 03:56
  • Indeed, there is no relation. If you look at the set of 200mm available for DSLRs and mirrorless cameras, you will see the minimum focus-distance varies between 50cm and 190cm. – Itai Apr 22 '16 at 04:00
  • Yes but can that distance (190cm for example) be reduced by moving the lens away from the sensor? That's my first question. My second is how far to move it by in order to achieve a particular distance. – James Apr 22 '16 at 04:03
  • The answers in the supposedly duplicate question seem to assume knowledge of the magnification. Is this calculatable given a working distance? – James Apr 22 '16 at 06:41
  • Lenses often quote magnification because it is easier to compare. It is a function of angle-of-view and minimum focus-distance which are both determined a lenses construction. – Itai Apr 22 '16 at 13:17

1 Answers1

0

Following a link in the answer provided by @Itai, I found this equation:

For:

  • focal length F
  • minimum focal distance X
  • extension L

Then the new working distance is: F(XF +XL - Lf)/(XL -LF +f^2)

I'm not sure about the validity of this equation or any of the assumptions that go into it.

James
  • 215
  • 1
  • 6