Originally posted by RobG I've tried some shots of a resolution chart and at least at a distance of a metre and a half, I can't see any sign of the drop in sharpness that appears in the landscape shots. The edge is certainly less sharp at 17mm focal length than at 50mm but at both extremes it compares well with the DA 16-45 ie it's sharper. I still don't understand the behaviour at infinity focus.
The behaviour is caused by infinity focus, but not by any fault of the lens. The closer areas in your image are out of focus because the zone of acceptable focus does not included them. You need to focus at the hyperfocal distance.
Please note carefully that I said "zone of acceptable focus", not "zone in focus". Acceptable focus, in the film days, was usually defined as the focus combined with an aperture that rendered a point that appeared sharp on an 8x10 inch print, when viewed from 12 inches.
There is an on line source for calculating depth of field, DOFmaster. I just popped out there, and the hyperfocal distance (bottom right corner of their screen) for a 16mm lens at f/8 is 5.3 feet. The "circle of confusion" is defined as 0.02 inches.
Try your pictures again. Move to 5.3 feet (1.6m) from something, focus and switch the lens to manual focus to keep that focus point, then take your picture. The trees will probably seem a bit out of focus in your viewfinder, but when you examine the picture on your monitor, you will be surprised at the better focus near to you.