0
\$\begingroup\$

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?
\$\endgroup\$
6
  • \$\begingroup\$ 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. \$\endgroup\$
    – Itai
    Apr 22, 2016 at 3:40
  • \$\begingroup\$ I understand that. I guess I naively thought that the minimum focal distance would be a function of the lens length. \$\endgroup\$
    – James
    Apr 22, 2016 at 3:56
  • \$\begingroup\$ 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. \$\endgroup\$
    – Itai
    Apr 22, 2016 at 4:00
  • \$\begingroup\$ 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. \$\endgroup\$
    – James
    Apr 22, 2016 at 4:03
  • \$\begingroup\$ The answers in the supposedly duplicate question seem to assume knowledge of the magnification. Is this calculatable given a working distance? \$\endgroup\$
    – James
    Apr 22, 2016 at 6:41

1 Answer 1

0
\$\begingroup\$

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.

\$\endgroup\$

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