I have a Sigma 150mm F2.8 EX DG OS HSM APO Macro lens mounted on my Canon 6D. Even if AF mode is set to minimum-to-infinity range, if my subject is way out of focus (too far or too near), like, say, ring is set for 60cm and subject is at 6m, the lens will not even attempt to auto-focus. If I manually rotate the focus ring a bit so the image will start to become a little sharp, the AF will grab the focus almost instantaneously.
This does not happen in any of my other lenses, but none of them is macro. Is this the intended behavior so I'll have to live with it, or is it a malfunction so I can claim my warranty?