I performed a test with my Sigma 12-24 and A900:
I tested at 12, 15, 17, 20, and 24mm at F4.5, 5.6, 6.3, 7.1, 8, 9, 10, and 11. I turned Image Stabilization off. I used 2 sec. timer drive mode with tripod. I put three pieces of paper on a wall. The pieces of paper had large text written on them. I then looked at all the images and saw the normal softer and more out of focus images from F4.5 through F8. I did not seem to see that one side (left) was more out of focus than the other - it's possible, but if so, it would be hard to measure. I also noticed that from 17 - 24mm there was less distortion - so, these seem to be all the standard issues with a 12-24mm lens.
So, based on the results of my test, would you say that when the new Sigma 12-24 lens comes out, it would be worth it to trade in my current 12-24 for it? I remember asking you if it was worth paying $1899 for the Zeiss 16-35 - and at the time, I made the decision to keep my Sigma 12-24. Now, the new Sigma might be under $1000, but will the performance of the new Sigma be worth it to trade up? Maybe this is a "dumb" question to ask, but I was hoping you would help me out with this question.