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, 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?

In this article "Fast full-field modulation transfer function analysis for photographic lens quality assessment " published in  Applied Optics , I describe how to do this and what the results are like. The first link here is to the open access version of the article.

Needless to say that I use this type of incorruptible inspection routinely as part of my checking. This type of work quickly adjusts your perspective of what can be expected and what you easily note in the field when not hinted by the measurement.