neuroanatomist said:
My iPhone isn't a place to judge sharpness, but if it's noticeably softer in one corner than the others, that's a sign of decentering. But often, an even easier way to tell is to shoot a clear blue sky or an evenly lit wall at max aperture, and look for asymmetrical vignetting.
The first example to me indicates that the lens in question is very badly decentered. What Neuro had to say about checking vignetting should suffice in that case. Decentering usually appears different at different zoom-extensions.
For a more subtle check, put your camera on a tripod, f5.6, live view, RAW, put a well outlined object at infinity into the center, focus manually, do not further change focus, then place object into all four corners at the same distances to those corners. If the lens is decentered, then in result there should be some obvious diferences between the corners in terms of sharpness, detail, aberrations. Deactivate all lens corrections in the RAW converter and apply only minimal sharpening if any. It's best to cut these corners and put them side to side in Photoshop into a single image, then check 1:1.
Do this process and comparison in the extreme focal ranges as well as in mid-focal-range.
But make sure not to choose backlit or conditions when thermal air movements may have an influence.
Best wishes
