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How do you fix / fine-tune the focus on a zoom lens? Nikon states

The D850 can fine-tune autofocus automatically. The results can be used with all lenses of the same type.

But does it also mean that fine-tuning one zoom lens in one specific focal length and aperture will fix focus problems for all focal lengths and apertures? (ie. calibrating a 16-35mm at f4 16mm will also provide proper calibration at 35mm f9?)

What about distance to the calibration target? Does it play any influence if you calibrate at a particular distance but then shoot your subjects at another?

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Unfortunately, as you've found, Nikon's in-body autofocus adjustment doesn't allow for different values at different focal lengths, apertures, or focus distances. Your suspicion is correct that those parameters can all change the adjustment required for perfect focus.

A common example is a phenomenon called focus shift: lenses with fast maximum apertures can experience changes in the plane of focus when stopping down from the widest apertures. Since the camera always focuses with the lens wide open and then stops down to take the picture (if set to a narrower aperture), lenses that exhibit this flaw may focus perfectly wide open but appear to need a slight adjustment when stopped down.

For in-body adjustments there's no ideal solution, but by calibrating the lens for the focal length and distance you'll be using most commonly, you can obtain an accurate calibration for those shots and hopefully be not too far off for the rest. It's worth mentioning that some third-party lens makers, notably Sigma and Tamron, sell docks that can be used to configure and calibrate their lenses, and those do allow independent adjustments at several different focus distances.

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    thank you sir, that is a correct and most complete explanation!
    – Zappa
    Apr 30 '20 at 11:45

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