I had cataract surgery on one eye the other day, and the results are a bit strange. If I take off my glasses, I have just fine distance vision through the relensed eye, but I can't see up close. If I leave my glasses on, I can see up close through the other eye, but get a mixture of the blurred image (through glasses and new lens) and my old glasses aided other eye.
Naturally I tried taking out the glass from the side with the new lens. Now I had two semi-sharp images, but they were rather drastically different in size. The difference in focal length between my spectacles and the inserted lens produces a dramatic difference in image size.
Too big, as the physician predicted, for my brain to merge them effectively.