# extention tube calculations? help needed.



## hamada (Apr 27, 2013)

hi, i bought a set of extention tubes for my macro work.

i need someone who can to explain to me how to calulate the new MFD and magnification.

for example when i put a 25mm extention tube on my 90mm macro that has 29cm MFD and 1:1 magnification... what is the new MFD and magnification?

or what if i put a 31mm extention tube on my 70-200mm f4.

is there maybe a good website that explains how to calculate this?


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## J.R. (Apr 27, 2013)

Try this ...

http://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm


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## TW (Apr 27, 2013)

Simply divide the amount of extension by the focal length pf the lens. 25mm ext/ 50mm lens = 1/2x, or 50% magnification. Also, just take a pic of a ruler and do the math!


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## Canon-F1 (Apr 27, 2013)

TW said:


> Also, just take a pic of a ruler and do the math!



yeah why not invent the wheel again. :

sure much easier to do that for a dozend lenses * 3 extention tubes then searching a formular. 




> http://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm



+1 

was going to post that.


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