Due to a lack of time, I asked Perplexity (what I prefer since several years now over ChatGPT) why Nikon's Z system still offers AFMA and Canon's not, and explained in my prompt our specific AF problems with our Z8 & Z 600mm f/6.3 PF combo (with and w/o 1.4x TC). Perplexity's answer might be interesting for some nerds (like me) here:
Nikon Z cameras still offer AF fine-tuning because mirrorless autofocus is not automatically immune to optical tolerances. The camera can focus on the sensor plane very accurately, but a given camera/lens/teleconverter combination can still show small mechanical or optical offsets, especially when a teleconverter is added and the effective aperture gets smaller.
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Why the adjustment exists
The main reason is that focus accuracy is not just a software problem; it is also affected by lens design, manufacturing tolerances, adapter/teleconverter alignment, and how the camera interprets phase-detect data. Even in mirrorless, the system can be “close but not perfect” for a specific lens combination, so Nikon keeps fine-tune as a correction layer for edge cases and for users who need maximum precision.
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With long lenses, wide apertures, and teleconverters, the depth of field becomes very thin, so even a tiny offset looks like obvious backfocus or frontfocus. That is why a birding setup can seem fine in general use but still miss consistently on small distant subjects.
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Nikon vs Canon approach
Canon’s RF mirrorless system is generally more self-correcting in practice because Canon has leaned heavily on on-sensor phase detection and lens communication designed as a tightly integrated mirrorless platform. Nikon’s Z system also uses on-sensor phase detection, but Nikon still exposes AF fine-tune because it allows compensating for residual per-lens/per-body/per-converter variation rather than assuming every combination is perfect out of the box.
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The key difference is not simply “Canon has dual pixel, Nikon does not.” It is more about how each manufacturer implements lens-body calibration, predictive AF behavior, and tolerance management across the whole optical chain. Canon’s system tends to hide more of that from the user; Nikon gives the user a manual correction tool when the combination is off.
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Your specific setup
A Z8 plus Z 600mm f/6.3 PF plus TC 1.4x should normally focus well, so a “massive backfocus” points to an out-of-spec combination, a teleconverter/lens alignment issue, a body/lens calibration mismatch, or a focus-shift behavior that becomes visible only at the longer effective focal length and smaller maximum aperture. The fact that the issue appears both with and without the TC suggests the base lens/body calibration is already off, and the TC may be exaggerating it rather than causing it alone.
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Practical interpretation
So the short answer is: Nikon keeps micro-adjustment because mirrorless AF is not mathematically perfect for every optical combination, and Nikon chooses to let users compensate for residual system error. Canon’s RF system often feels more automatic because Canon’s implementation relies more on integrated on-sensor focusing and hides calibration differences better, but it is not because mirrorless AF fundamentally cannot need calibration.
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A useful rule of thumb is that if one Nikon Z lens or lens-plus-TC combo needs a large correction, that is usually a sign to test the lens, TC, and body separately, because the camera is compensating for a real mismatch rather than a normal tuning quirk.
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My personal conclusion: looks like we'd have to send this combo to Nikon's service because of too big production tolerances - typical Nikon's lousy quality, I have to say after many years of using Canon and Nikon gear side by side
: but if there is a hunter around they both are trophy...
