taking some shots today, with my new lens(Tamron SP70-300mm F/4-5.6 Di VC USD), and suddenly got really worried that it wouldnt step less than F5.6. I couldn't figure out why, the dial refused to do something it should, and I thought bugger it's stuffed! Got on the net, and of course I found someone with a similar issue, and they sent theirs back. I was bummed at that stage, for me this was a big purchase, and I didn't look forward to returning it via the post (online sale). So anyway, I sat down after lunch, and had another fiddle, and all of a sudden it would step to f5! I thought, hmm maybe this is an intermittant issue? I zoomed the lens to 300, and the problem came back, wont step less than 5.6. Then I had the crazy idea that focal length could have something to do with it..............and YAY at 70mm I can F4, and it changes all the way through to 300mm. I am not clear of the why/how of it yet, but......I am so happy the lens isn't stuffed!