A little background:
I bought a Sigma 17-70 macro lens from an Ebay seller that exhibits consistent back-focus at 17mm-19mm when used fairly close to any given subject.
Even under good lighting conditions with a subject high in contrast, the lens will consistently fail to find focus at apertures as high as f/8.0. Multiple tests have confirmed this. Unfortunately, the seller refuses to believe me and so now I have filed a case against him.
However, when writing up my case, I made two big mistakes:
1. I safely assumed that any range between 17-24mm would exhibit back-focus, but upon further testing, I found this to be untrue.
Bizarrely enough, the lens will only back-focus right at the 17-19mm range, with 17mm exhibiting the most obvious problems.
2. At the time I wrote the case, I had only tested the lens at an aperture of f/2.8. (Wide-Open) Thus, I claimed that only larger apertures in conjunction with the proper focal length would make the issue appear. This is also not true.
The lens actually does continue to back-focus all the way up to f/8.0, as stated previously.
So now I ask:: What am I to do about this? One of these errors is actually "in my favor" so-to-speak, as the inability to focus properly manifests at apertures much higher than originally anticipated.
On the other hand, my claim of the lens backfocusing close to the 24mm range is an exaggeration.
What would be the honest thing to do here? Do I just let Ebay continue to review the case and let them in on my recent discoveries AFTER they contact me? Or do I contact the seller and admit my miscalculation *now?*