Canon EF 100mm f/2.8 L IS USM + extension tubes

Has anyone used tubes with this lens?

No, but if it is anything like the USM version you will need 72mm of extension to get to 2x. All macro lenses that have internal focusing lose focal length at minimum focus. The USM version of that lens was roughly 72mm at 1x. Canon's EF-S 60mm is 37mm at minimum focus, so you only need 37mm of extension to get to 2x. IMHO, with lenses above 60mm, you are better off using a diopter or a teleconverter to get them above 1x.

General practice, even in the old days before AF lenses used 'tricks' like shortening actual FL of lens at MFD, was to use shorter FL for higher magnfication macro work

Extension length / FL of lens = magnification factor

so longer FL always requires more extension. That is why extension bellows were often used for high mag work.

Wilt wrote in post #19340159

Extension length / FL of lens = magnification factor

My point was that the formula you quoted does not work for macro lenses. The only reliable way to determine magnification is to set your lens to its minimum focusing point, photograph a metric ruler, and then divide the width of your sensor by the number of millimeters in the image.

chuckmiller wrote in post #19340136
Has anyone used tubes with this lens?

Yes, I regularly use a 25mm tube just to get a little bit more magnification. It makes a difference for jumping spiders etc.

S.R.M. wrote in post #19340655
Yes, I regularly use a 25mm tube just to get a little bit more magnification. It makes a difference for jumping spiders etc.

A Canon brand tube or another brand?