Lens variations

Joe Moche wrote:

A common theme on many of these forums involves folks who argue over the sharpness of a particular lens. The argument often ends with "You got a bad sample. Send it back." Honestly, there seem to be an awful lot of "bad samples" floating around out there. Is the lens manufacturing process really so loose that a high number of bad samples would be produced? Are bad samples just an aberration, or is there a range of sharpness (from great to poor) that affects all lenses? Do lens manufacturers test a lens before boxing it up, and is there some minimum level of performance that renders the lens "good enough?" Finally, how can one know if a newly-purchased lens is a bad sample, other than by making a totally subjective assessment by comparing results to results claimed by others?

I have neither the time, skill, or desire to run lab tests on every lens I buy,

It's not that hard to do. I can do it in ten or fifteen minutes. The first time you do it, it'll take you longer, but soon you'll be as fast as I am.

and if a lens doesn't meet my expectations, I have no problem sending it back. That said, I see no reason to settle for "bad copy" when a better copy can be had for the same price. Thoughts?

The most common sample variations seem to result in asymmetrical sharpness variation. A well assembled lens may have aberrations, but those aberrations will be radially symmetric.

I have devised a test for that kind of asymmetry:

https://blog.kasson.com/lens-screening-testing/