# 180 mm macro with kenko extension tubes



## chauncey (Jun 30, 2014)

1Ds3 & 180 macro, coupled with extension tubes at minimum focusing distance...12mm/20mm/36mm, all or any combination thereof. 
What are the formulas to determine magnification factors using the various extension tubes and/or combinations? :-[


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## Mt Spokane Photography (Jun 30, 2014)

There is some information here. The formula is not simple.
12mm ET - 1.09X
25mm ET - 1.21X
A TC will get you more magnification, you can use 1.4X or 2X.


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## tolusina (Jun 30, 2014)

There's a pretty simple formula on this page..........
http://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm


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## Mt Spokane Photography (Jun 30, 2014)

tolusina said:


> There's a pretty simple formula on this page..........
> http://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm


 
For a short lens, its going to be pretty good, but for a longer one, we need to know where the nodal point is inside the lens, it gets rather complicated.

That formula gives 1.14X with a 180mm lens, and 25mm tube, but its really 1.21X. The error becomes apparent with a longer lens.


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