Originally posted by johnyates Best distance is the one you generally shoot at.

+1

It is typically not possible to find an AF micro-adjustment that works for all focus distances, and in the case of a zoom, for all focal lengths.

That's why Sigma's lens dock allows you to calibrate different distances and focal lengths individually. If one AF adjustment value were fine for all combinations of focus distances and focal lengths, Sigma wouldn't make you fill in a 2D matrix for all combinations.

Unfortunately, for some lenses, the optimal adjustment also depends on the f-ratio used (due to a phenomenon called "focus shift").

So while wide-open, and the highest focal length would make it easiest to see the focus error, optimising the AF adjustment value for this case may mean that you may not get the best results for your most frequently used f-ratio and focal length. A similar argument can be made for the distance. What good is it, if you optimise a portrait lens for infinity?

You may want to look at the

AF adjustment hints I compiled.