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• ## Re: Calculations with extension tubes

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• ... Yes, most likely. ... No, not a general one. It depends on the lens what happens. Contrary to common believe FoV does not exactly depend on some specific
Message 1 of 3 , Sep 24, 2010
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> When using extension tubes for close ups does that affect the FOV of the lens?

Yes, most likely.

> If so, is there a formula for that?

No, not a general one. It depends on the lens what happens. Contrary to
common believe FoV does not exactly depend on some specific parameters
like focal length and sensor size. The common formulas to calculate FoV
from Focal length and sensor size are approximations only that give
reasonably good results for standard photograpy but not for close up
work. For details read Rik Littlefield's paper (PDF):
http://tinyurl.com/d29lu

> Is there also a formula to calculate how a certain extension would affect the point of the closest distance?

More or less. If you have a classical lens which is focused only by
moving the entire lens the length of the extension tube is simply added
to the focus extension. You can use the thin lens formula:
1/f = 1/S1 + 1/S2 where f is the focal length, S1 is the distance from
the lens to the sensor and S2 is the distance from the lens to the
subject in focus. If the lens is focused to infinity, S1 equals f and
1/S2 is zero.

The complication arises from the near focus usually given as a distance
from the sensor plane. And usual lenses are not thin lenses. They have
two principal planes with a distance. Exact calculations are only
possible if you know this distance.

A very good starting point is the forum on http://www.photomacrography.net/

--
Erik Krause
http://www.erik-krause.de
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