I believe one of the other members using FoCal found that the increased resolution of the D800 allowed it to show more focus error - the center was so sharp that it allowed a higher level of resolution - and normal performance at the edges was worse in comparison to the center.

I don't think FoCal is ready to support using the numerical values to compare cameras or lenses. But it should give you a good indication of performance and how fine tuning should be adjusted.

There is definitely an art to interpreting the results. FoCal produces a lot of information and is a good tool. Having a bit of context from different lenses and camera bodies should be helpful.

Eric Bowles
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