# How much does extending affect range of focusing distances?

It's well known that moving a lens away from imaging surface will decrease both minimum and maximum focusing distance. There are times when it would be good to know how much those distances will change.

Is there a formula to estimate the range of available focusing distances (i.e. minimum and maximum focusing distance) with a given lens after adding an extension tube (or stretching a bellows)?

• Mar 17, 2012 at 20:17

For minimum distance calculations, see here for details: http://photo.net/large-format-photography-forum/00Sswe

Extract:

"The core formula [for minimum focus distance] is 1 / f = 1 / u + 1 / v

Where f is the focal length of the lens, u is lens to subject distance, and v is lens to film distance.

More properly, v and u are measured from the focal point of the lens. For a symmetric lens, that's usually smack-dab in the middle of the lens. For a telephoto lens, it's way in front of the lens -- that's the definition of telephoto. For a retrofocus lens, it's way behind the lens.

Magnification is v / u."

[add your extension tube length to 'v' to see how the range changes.]

More calcs of all kinds here: http://photo.net/learn/optics/lensTutorial#part1