I just (as in tonight) got a set of Kenko extension tubes. When I was looking for them I wondered what I should expect out of them: how much magnification would there be? Well, I found some complicated formulas that assumed I knew details of the distances between lens and sensor and other stuff. It was not helpful. I couldn't find a simple reference photo.
So, for the benefit of anyone who might be interested: here is a set of images going from max magnification (all three tubes together) to minimum (shortest tube only) and a reference image with no tubes. All were shot with the Canon 85mm F1.8, at F1.8 using a Canon 6D. All were shot at the minumum focus distance for maximum magnification. The vertical dimension of the image is uncropped as shown.
Kenko Tubes with 85mm F1.8 by MCG Photo, on Flickr
Top left is all tubes together.
Top right is intermediate.
Bottom left is the shortest tube only.
Bottom right is with no extension tubes (shot at F8).
Hope this is useful to someone.