I shall soon visit my Dealer and shall test the Canon, Sigma and Tamron.
I am using my Personal Test which I call "Brick Test"
The Dealer's Building has Bricks Wall so I shoot the bricks 18,50,100,135,200
with Min F and check sharpness corner to corner.
I shall post my Test Results.
I bought and tested all three "uberzooms:" The Sigma 18-200mm OS, the Canon 18-200mm IS and the Tamron 18-270mm VC. My observations:
Sigma: Good build, speedy and quiet AF, OS works well. Sharpest towards center, middle of zoom range is the worst, good at 18mm and 200mm.
Canon: Decent build quality, AF is speedy if a bit noisy. Sharper in the corners than the Sigma, but not quite as sharp as the Sigma in the center. Worst performance at 18mm, good across the rest of the range.
Tamron: Probably the least sharp of the three, but only by a hair. Good performance across the range, loses sharpness a bit at 270mm. Build seems a bit cheaper than the others, but AF is quick and quiet. VC works well.
Overall, any of the three will work well. I decided that the image quality differences between the three weren't really all that different, and all needed to be stopped down to about f/8 for best performance. These aren't lenses you buy for sharpness, or shallow DOF. You buy them to have as an "all in one" lens that you can stick on the camera and gain a lot of flexibility from. I have sold the Canon and am selling the Sigma. In my case, I wanted to get the maximum focal range from a lens like this, so that was what I based my decision on. As long as you understand the compromises that are required to design and build a lens like this, you will enjoy it.
