At least in the case of the Maxxum 7/Sigma lens incompatibility, it had nothing to do with the AF system. Before it was re-chipped, my Quantaray (Ritz brand name for Sigma) 135-400 mm would lock up the camera if it was metering (display lit) and the lens was set at or zoomed through 150mm, 200mm, and 300mm. As long as I preset the zoom to something else when the display was dark, I was ok. However, it did cause me to miss a number of shots. Once the camera was locked up, the only way to unlock it was to turn it off and reset the lens to a safe FL, and then turn it back on.Greg Beetham wrote:There's nothing to say you can't use MF with any Sigma lens, just turn AF off, save some battery power and focus manually, with a little practise you'll find it's easy to do, and you won't ever have to worry about breaking anything, it's a small price to pay for in some cases very fine optics, you might have to set the shutter button option accordingly, from focus priority to shutter priority.
Greg
ps. I'm assuming of course that the incompatibity issue is related to AF, I haven't heard of any other particular Sigma problem regarding compatibility with Sony cameras, except maybe A-mount lenses not focussing on the NEX, but all A-mount lenses are in the same boat there regardless of brand...no great biggie really, again just use MF.
Also, Canon has had their share of Sigma lens incompatibilities.
tom