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If I know the focal length of a lens, how can I know the minimum focus distance? I know there is mark in the lens, but if I don't have the lens how can I know the MFD?

I have checked several webpage, but I am still wondering whether there's a formula?

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    The manufacturer's web site will usually list the specifications for their lenses, including minimum focus distance. – coneslayer Aug 17 '15 at 15:23
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It depends entirely on how the lens was designed, there is no general formula or way of determining the minimum focus distance. Lenses of the same focal length can have completely different minimum focus distances depending on which factors the lens designer chose to optimise.

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There is no direct relation between focal length and minimum focal distance.

Usually a long focal length means a long minimum focal distance, but that is just because a lens built that way also has other characteristics that are preferable, like having a reasonable focus range.

If you add extension tubes between the lens and the camera, the minimum focal distance will be shorter. That's because you change the characteristics of the lens. With enough extension tubes you can even get a negative minimum working distance (the distance between the front of the lens and the subject), i.e. it can focus behind the front lens element. That shows that you could easily build a lens with any focal length and a very short minimum focal distance, but it would also have a very short focus range.

  • The focal length of a lens doesn't change when adding an extension tube. – Matt Grum Aug 17 '15 at 16:44
  • @MattGrum: Are you sure? You are moving the lens elements away from the film plane. – Guffa Aug 17 '15 at 16:48
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    But you are not changing the way the lens bends light. And you are not changing the distance at which colimated light is focused, it just means that colimated light is no longer in focus by the time it reaches the film/sensor plane. This is why adding extension tubes usually means you lose infinity focus. – Michael C Aug 17 '15 at 17:32

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