Does anyone know how to calculate the theoretical magnification one can get from extension tubes. I just bought a set and they're great fun but the instructions were in computer translated Chinese so I couldn't make head or tales of them. There was a table of sorts that seemed to suggest that if I was to use all of them together I might get 1.5 magnification at best although I suppose it depends both on the focal length of the lens and how close it will focus.
In the last picture in this set:

I've combined my 60 mm macro, that gives a 1:1 magnification in itself, with 3 extension rings. The flowers are a diminutive 5 mm across and the sensor of my 400D is 22.2 x 14.8 mm so it would seem, in this case, where the flower fills up the image on the short side, I'm getting about a 3x magnification.
So, to recap, is there some clever way of calculating the magnification in advance?
The short answer is no. It also depends on the length of your tubes. It's worth looking at the manual that came with your EFS-60 and sees what is says about Canon "extenders". If your tubes' length is the same as theirs, you should get an approximate answer.

This post:


Offered some interesting explanations. The third to last comment seems the most technically proficient.
I worked out the ratios for my set and calculated a magnification up to 8X for 68mm of extension. "Calculated" isn't perhaps the best word, as it was all trial and error rather than having any equation to work through. I've never seen a formula published and imagine it would have too many variables to be of practical use. I guess, you'd need to take focal length, extension length, sensor size and perhaps some other attributes into account.

I'd advise working through like this for every combination of tubes you have. I can't say that I can remember any of the numbers, but it gave me a lot better feel for what tubes to select in any given instance.

Good luck with it.