Originally Posted by shep315 
If this is the wrong place to ask please let me know. I have a Canon 100mm F/2.8 Macro and was looking to purchase a EF 25 II extension tube. Will the extension tube help in magnification much? ThanksHi,
Using the ext tube at minimum focus would increase the magnification to around 1.36:1 from 1:1.
ie a 1cm long subject would have an image on the sensor of 1.36cms instead of 1cm.
It’s better value if you just buy a kenko canon fit ext tube set of 68mm in total. That will allow you to get to just over 2:1 magnification.
Brian V