17-40L with Kenko Ext. Tubes

Extension tubes will work pretty well with just about any lens. The magnification you get is just dependant on the length of tubes and focal length of the lens.

Mag = Length of tubes (mm)/focal length of lens (mm).
So for a 50mm lens with 68mm of tubes, mag = 68/50 = 1.36:1

There is a limit somewhere around 28mm focal length where the focal point is so close to the lens as to make it impractical.

Brian V.

thanks for the formula.
what happens to the working distance?
and does this assume that you are not already at 1:1 to start? e.g. - if you have the 100mm f2.8 macro canon then adding the tubes would put you under 1:1....or is that a 1.47 magnifaction factor? (heheh only 4-5 stacks of those babies and you get a cheap mpe65! just kidding!)

man....photography sure can be confusing! lol