D600 AF, Hitting the Limits, or Faulty?

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D600 AF, Hitting the Limits, or Faulty?

It's a D600 with a Tamron SP 45mm f/1.8. Calibrated with 'green dot' method for all three distance ranges with Tamron TAP-In console. The AF does OK for accuracy, wide open on close-up objects and things like headshots, but gets very mixed results at focus-critical apertures (1.8-4) on smaller objects at 6'+.

Attached examples show an overlay of the single autofocus box, set in the center, where it was confirming focus through the viewfinder when the photos were taken. These are heavily cropped to roughly the center 1/6th or less of the entire frame, to show where critical focus landed. It looks like backfocus error, but if I actually dial in enough AFMA to fix it, I end up with gross frontfocus on larger more obvious AF target objects, even at 6'+.

The last image shows some improvement, although slight. It seemingly got a little better after using an air blower on the AF unit in the mirror box. It's really hard to reach, and I can't get a stream of air directly on to it. So before I try to go through the trouble of figuring out how to get more air, or an Eclipse wipe, or something, onto the recessed AF surface in the bottom of the mirror box, I need a sanity check...

At this distance with small objects, it seems almost like the system wants to grab focus on objects that are outside of the center AF box, and behind whatever's inside the box. It's the same no matter if it's 1-point, 9-point, C-AF, 3D Tracking, backbutton or half shutter press, etc. I am using S-AF single point here to demonstrate.  Also, using the live view contrast detect on these same subject nails focus just fine.

I go back and forth on these ideas:

Something (maybe some of that dust or oil from the shutter that made the D600 so infamous) is physically impeding the ability of the AF system to 'see' smaller/more distant objects clearly.

The D600's AF unit just doesn't have the 'resolution' to reliably get focus on small/distant objects that fall mostly inside an AF box.

What's the more likely explanation here?

D600 Center Crop, 45mm f/1.8 S-AF

D600 Center Crop, 45mm f/1.8 S-AF

D600 Center Crop, 45mm f/1.8 S-AF, after mirror box dusting.

ANSWER:
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Nikon D600 Tamron SP 45mm F1.8 Di VC USD
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