just a small vent as I head out to do another day's shooting. I've had two Sigma "heavies" ever. The 50-500 and the 120-300. Both are heavy lenses. The 120-300 is always on the monopod. Both have had issues with the screws at the mount end loosening with normal use. Eventually after numerous time re-tightening the screws, the 50-500 suffered a catastrophic failure where the mount snapped off completely. The screws didn't break, but the tabs they secured broke off. I got home last night from shooting all day and the mount on the 120-300 was loose. PITA! At least I knew what to do to tighten it this time around, so I set about retightening those three tiny little screws. Well that went fine till I had to replace the fragile little gaskets (for want of a better description). They broke, one of them into three or four bits. Luckily none fell into the lens. I ended up having to go without them. Then I dropped one of the four screws that hold the nice shiny base plate over the whole lot. Crap! Can't find it after it hit the floor and went God knows where. I guess I'll just have to put up with it until I can get it repaired by someone not as clumsy as me. At least it still works.
What I can't get over though is what a pissy little design they have for holding the lens onto the mount, or vice versa. The torsional forces those little screws have to resist in normal use are huge, I would have thought. Anyway, that is my vent for the day.
Oh, and the screws on the Sigma 1.4 TC also loosened of their own accord and I had the not only re-tighten those, but the centre cylindrical part also rotated loose and had to be twisted back tight again. None of this happens on my Canon lenses.