Today I got my 70-200 back from Canon (the focus was slipping). As an experiment I tried it out with my extension tubes. It doesn’t give as much magnification as my 50mm prime (I think) but it focuses from further away and has IS, making handheld macro a bit easier.
Interestingly however, the camera still really struggles to autofocus with any extension tubes attached, which suggests that it is definitely the camera’s autofocus mechanism that is letting it down. With such narrow depth of field, you really want to be manual focusing most of the time anyway.
I shot this in front of a mirror with the flash bounced at the ceiling.
Lens: 70-200mm f/4 L IS
Focal length: 160mm (with extension tubes)
Shutter speed: 1/60