extension tube?

[t5Nitro]

Has anyone ever attached an extension tube to a 24-70? How much closer would I be able to focus?

[smb2]

Not specifically on that lens or a zoom for that matter.
Answer depends on the size of the extension tube. I have a set, 12mm, 20mm, 36mm from Kenko and they work great. The longer the more challenging because the DOF becomes minuscule. At the 24 end of your zoom I think you would have poor results but the 70mm end might work.
The Kenko set handles all automatic functions but I have only used them with a 105 and 60 macro lenses.
Go to your camera store or borrow a set and try it out.

[brent ward]

I think that lens highest magnification is at 35mm, so your closest focusing difference will be very close.