I agree with everything you say here, for sure. People are far too quick to claim "bad copy" without appropriate testing. But there's no doubt there are bad copies out there, of most lenses, and that certain lenses or manufacturers have more than others.
I have bought many Nikon lenses and never had a bad copy. But I know people who have definitely had some bad ones. On two occasions I've had a bad copy of a Tamron lens, and most recently I got a bum copy of the Zeiss 100MP. I proved that it was bad and it was replaced. If I suspect a bad copy, I do very throrough testing, with focus charts, on a good tripod, using a remote trigger and mirror lock-up.
Originally posted by Lowell Goudge I personally think the biggest issue with sample variation is immediately behind the viewfinder.
While I don;t claim to have perfect luck, clearly I have not won the lottery yet, I have never had what I consider a bad copy of a lens. I have generally looked at the lenses I have bought, and the images they produce, with the methods I use the lenses under, and accepted the results for what they are.
Today, with computers and huge monitors we have the ability to look at images microscopically, and many of the Sample issues, aside from decentering which fortunately I have not had, are the result of technique not product.
To properly consider sharpness and image quality generally requires use of a tripod.
The old rule of thumb 1/focal length, for film or 1/focal length/1.5 for digital really only applies to looking at an image in 8 x 10 format, and not looking at it zoomed in to the pixel level, the definition of acceptable sharpness was based upon the same circles of confusion that Depth of field was based upon, although applied a little differently, and therefore does not apply to pixel peeping at what would equate to a 10 foot high print,
We need to get serious about this, if it was not shot with a tripod or flash, the person behind the viewfinder has much more to do with lens sharpness than the lens.