I just received my Sigma 30mm/2.8 from B&H and when off the camera the lens has a deep rattle when shaken, like something's loose inside. It seems to perform ok but is this normal for a Sigma lens? I've used many other brand auto-focus lenses and never had one rattle like this.
It is normal ... It is also mentioned in the literature from the manufacturer which came in the box with the lens. Everyone who bought this lens (including me) freaked out about it at first. It is just a side effect of the way Sigma designed the AF motor in the 30mm and the 19mm. When the lens is mounted and the camera is on it stops making the noise.
I am sure Sigma could have designed the 19mm and 30mm without this AF component rattle, but you only really hear it when the lens isn't mounted and it's hard to complain too much about a brand new lens with excellent sharpness that only costs $200.
Yep, it is the linear AF. I agree that a rattling lens is weird and doesn't promote confidence in the build quality, but the performance of this lens is GREAT for a $200 lens. It is like that old business saying (which I am paraphrasing), "You can have the best performance, the best build/style, and the lowest price ... but you have to pick only two."
The rattle is easy enough to live with. I should have checked the manual but who reads manuals for lenses. And when I noticed the rattle, I didn't have the manual with me. The lens is a nice performer though. All the reviews about it being sharp corner to corner wide open are true. I like the focal length and love the price.