I have a Pentax K100D (which I LOVE) and it came with the 18-55mm kit lens. I purchased a Sigma DG 28-70mm lens for it about a month and a half ago and it worked wonderfully. Now when I put it on the camera the LCD Panel (not the playback screen) flashes AV at the top and it won't do anything. I'm getting a little frustrated and I don't know what to do. The batteries are brand new and I don't think I changed any settings on the camera and I don't know if maybe someone else did or what. Does anyone have any suggestions? Please help! Thanks so much!