Heidfirst wrote:sticky aperture blades not stopping down properly?
Very probably. I had a similar fault on a Sigma 50-500 (which to be fair, had been dropped!). Sent it back to Sigma for servicing via Warehouse Express (from whom I bought it) and it worked well for a while but then the fault began to recur. I finally cured it by using a tiny
quantity of silicon lubricant on the actuating arm which protrudes from the lens and engages with the mechanism in the camera body. Pretty drastic and definitely not recommended unless one is well versed in such mechanical matters (and has a steady hand) but the lens has performed perfectly ever since (about 14 months ago and probably well over 10,000 frames).