Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
processor Contributing Member • Posts: 648
open loop and AI Servo

Ben Strasser wrote:

you would never be able to have AI Servo focus.

Open loop, when properly calibrated, does AI Servo as good as closed one, only faster.

That's the beauty of the phase detection AF system - it can calculate the precise focus position based on single measurement - the shift between the curves on half-pair AF sensors. In normal AF mode, it takes one more measurement along the way to refine the calculation. In AI Servo mode, the prediction of subject distance changes is even more important than the refinement. All this results in very agile AF and good accuracy when AF system is properly calibrated.

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