As you might know, different copies of the same lens can vary in quality, and some people go as far as to purchase multiple copies to pick the sharpest one before returning the others. Roger Cicala over at LensRentals wanted to quantify exactly how much variation actually exists between copies of the same lens, so he subjected some to Imatest quality tests:
[...] while the Canon 100mm f/2.8 IS L lens is a bit sharper than the other two on average, not every copy is. If someone was doing a careful comparative review there’s a fair chance they could get a copy that wasn’t any sharper than the other two lenses. I think this explains why two careful reviewers may have slightly different opinions on a given lens.
That’s interesting to think about. Two highly objective reviews of the same lens could come to different conclusions about relative sharpness compared to other lenses, simply because there are differences among copies of that lens. Too bad reviews are usually based on a single copy of a lens, rather than the average performance of multiple copies.
Notes on Lens and Camera Variation [LensRentals]