how to figure out ratio with ext. tubes?

The general formula for finding the magnification ratio is:

A bit more usefully, you can find the magnification ratio with a lightly modified version of the formula:


So, using Google to find the maximum magnification of the 50mm f/1.8 to be 0.15x, we can use the second formula to calculate the maximum magnification of the Nifty Fifty focused to its closest point and 68mm of extension tubes:

Therefore, your maximum magnification with the Nifty Fifty and a the full 68mm of extension tubes is 1.51:1, or 1.51x.


Another way to find your total magnification is to photograph a ruler; the magnification can then be found by dividing the Camera's sensor's size by the length that you were able to capture in the frame.

So, knowing that the 40D you are using has a sensor size of 22.2mm horizontal, you take a picture of a ruler, and see that you can fit about 14.7mm horizontally across the frame. You can then find your magnification to be:


Again, we get 1.51:1 or 1.51x. This is the same answer that we had using the previous calculations, which means that the lens must have been focused to its closest point.


Neither of these methods is a perfect way to calculate magnification due to all kinds of manufacturing variations and other factors, but in my experience, the results have always been accurate and reproducible enough for my purposes.

Hope this helps!