I'm currently using the 24-105 @24mm on a 5D2 for landscapes. Stopped down with tripod, MLU, self-timer/remote and careful focusing, IQ is adequate for my purposes (stock, large prints). However, I prefer to go a bit wider (20/21) and if possible sharper across the frame. If cost were no concern I would get the Zeiss 21 or perhaps the Cannon 16-35. The Samyang 14 looks good but it's too wide. If they come out with a 21 that might be my best alternative but who knows if it will happen. My question is whether the Canon 17-40 or 20/2.8, or some non-Canon lens on an adapter, stopped down, would give up much in across-the-frame sharpness relative to the Zeiss. I can pay up for the Zeiss or the 16-35 but I prefer a cheaper alternative if it gives decent corner sharpness at around 20/21mm when stopped down. Thanks.