>So Ferguson, if AF fine tuning produces a ski slope going up >to the right and just about levels off before you hit +20, has >a setting of +18 or +19 actually corrected the focus >point...or just improved it?
Just from a math standpoint I would say if still clearly ascending it means you have insufficient correction to make it the best - but if it's leveling off maybe you are close enough?
>I had that happen with my 50mm f/1.8 and when I put the 1.4x >TC on the 70-200. The adjustment for the 70-200mm without the >TC was zero. > >Interestingly enough, the QoF with the TC on the 70-200mm was >actually a little higher than before I put it on (VR was off >for both) after I did the AF fine tuning with it on.
I wish we knew more about exactly what those values mean. For example, if they are simply a measure of how clear the edges are on the target, then the 1.4x is magnifying them and so may by its nature give a higher value. Kind of like a person looking at a sparrow at 100 yards through a 600mm lens versus as 60mm. Do we really know if it's measuring pure focus or does relative size of the target matter. I just got home from travel, I plan to do some testing, if I get ambitious I may try two targets in otherwise identical situation, with different scale of the targets. Got to figure out lighting first for different distances.