Macro Extension Tubes - Educate me

I recently bought a set of Polaroid Macro Extension Tubes, as the the reviews here on POTN were favourable and the price matched my budget - they really are a nice piece of kit.

I don't think there is a simple answer for that. For idealistic case of a thin lens, the usual lens equation applies:

1/S1 + 1/S2 = 1/F

Here S1 and S2 is the distance from the lens to the subject and to the sensor, respectively, F is the focal length. By adding extension tubes one increases S2, and using the above equation one can calculate the new value of S1:

S1 = 1/(1/F - 1/S2)

Unfortunately, things are not as simple with complex lenses, and especially with zooms. One usually has to test lens/tube combinations to measure the magnification and MFD.

You can use this calculator. http://eosdoc.com/jlca​lc/

Your inputs...

1. FL,
   
2. minimum focus distance for that lens without extension,
   
3. length of extension tube