Leaves contradictory impression

I own the lens for nearly two years and have used it in different situations from still life on the city to landscape and architectural photography. When traveling, it was the addition of Universal zooms in the extreme wide-angle range. Many shots with this lens are therefore taken with a focal length of 10 mm (on a Canon 50D). Therefore, the following explanations relate essentially to the use of the lens with a short focal length (under 12 mm).
In view of the excellent test results - especially in terms of high resolution down to the border area - though not all, but a number of recordings of a disappointment: in particular landscapes were very sharp in the middle, but the border area from about halfway between center and edge also when dimmed lens so blurred that the sharpness waste even stabbed at first glance on a printout in size A4 eye. I brought the lens to the service, and it was adjusted. Nothing changed in the issue. After a few hundred pictures more I realized what the problem is: the lens is at the shortest focal length extreme, distance-dependent curvature of field, that is close to the minimum focusing distance is the image field plan, are from a shooting distance of about two meters, when the center is in focus, objects in the outer regions of the image only in focus when they are much closer than the objects in the center. The greater the distance, the greater the curvature of the image field.
The problem can be get by stopping due to grips: If the entire screen at more distant objects will be in focus, it helps to select in the camera one of the outer AF focal areas and by reducing the aperture to 8 or 11, the depth of field as far as to increase that even the center is sharp. Focusing on the center of the image and then dim bring my experience nothing.
In practice this means that it is possible to make good photos with impressive perspective with this lens, however, only under constant attention of the extreme optical characteristics of the good piece. Somehow the impression is that the lens has been optimized for the test lab rather than for photography in the open field.
The autofocus works equally fast, quiet and accurate. The lens is relatively small and light and therefore compatible with the ergonomically cameras 100 series well. It looks solid processed, without play or wobbling of the moving parts.