A while ago Klaus posted a method for testing lens decentering. The problem is that I can't locate the thread. Anyone happen to have it in their bookmarks?
My own crude method is to shoot a far away cityscape from a high viewpoint, so that everything corner-to-corner is practically at infinity. If sharpness distribution is asymmetrical then I consider the lens to be decentered. The method Klaus posted relied on some test target that would potentially allow it to be performed at the store, right after (or during) the purchase of the lens. It's also more practical when meeting a seller of a second hand lens.
My own crude method is to shoot a far away cityscape from a high viewpoint, so that everything corner-to-corner is practically at infinity. If sharpness distribution is asymmetrical then I consider the lens to be decentered. The method Klaus posted relied on some test target that would potentially allow it to be performed at the store, right after (or during) the purchase of the lens. It's also more practical when meeting a seller of a second hand lens.