Is lens calibration required?
There is some suggestion that lenses that get a bit of physical abuse (bumps and knocks) do tend to go out of alignment.
The lens groups that are adjusted to give best resolution at the end of manufacture are by their nature a bit prone to movement. A knock can put it out.
Some anecdotal evidence suggests IS can wear over time and need repair or adjustment (some pins that wear out?).
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from Open Air Fashion Photography
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