Yes you read it right, the corners are brighter. I have the same affect when mounted the body on my 70-200 f/2.8, but the affect is more visible when mounting on my 500, maybe because of an f/4 lens? Basically there is a circular area that is about 75% of the viewfinder and this erea is slightly darker than the 4 corners. I did not notice it at all when I mounted it on my f/2.8 zooms, but when I put it on my 500 the affect is a bit more, enough for you to notice. All images come out normal.