Crossed Up Cross Sensors?

One thing I’m quickly noting is that suddenly I have to pay a lot more attention to which focus points are active on the D5, mainly because I’ve been trying things out with the 500mm f/4.

We’ve always had some limitations when you start using TCs, but now we’ve got limitations with some lenses, particularly many f/4 or slower ones. 

So here’s the short scoop: we’ve got 153 autofocus sensors. Of those, most of the time we can select 55, and most of the time 99 function as cross sensors. We have two things that change those last two bits: some lenses, and slower apertures due to TC use. 

Lenses

  • The 12-24mm, 60mm, and 600mm lenses all drop the outer columns as cross sensors. We’re down to 63 cross sensors, but we still have 153 sensors of which we can select 55.
  • The 200-400mm, 500mm, all f/5.6 or slower lenses, and all non-AF-S lenses drop all outer columns as cross sensors, leaving only the middle 45 as cross sensors. We still have 153 sensors of which we can select 55.

TCs

The TCs will tend to impact both the number of cross sensors available as well as the number you can select. I haven’t verified this yet, but the Nikon documentation basically says:

  • f/5.6 — TC-17E and 20E on f/2.8 lenses and TC-14E on f/2.8 or /f4 lenses: 45 cross sensors, 55 selectable
  • f/5.6 to f/8 — TC-17E on f/4 lenses and TC-125E on the 800mm: 25 cross sensors, only 37 sensors used, and only 17 of those are selectable (center nine and the horizontal row)
  • f/8 — TC-20E on f/4 lenses and TC-14E on f/5.6 lenses: 5 cross sensors, only 15 sensors used, on 9 are selectable, and only the central sensor is cross

So the short version is this: If you’re not using one of the exotics or a TC, everything operates as you’d expect, as long as the lens is AF-S and not the 12-24mm or the 60mm f/2.8 Micro-Nikkor. Outside of that, you’re going to have to pay some attention to what’s going on, as the system will get more restrictive.

Why the limitations? Well, Nikon is pushing the D5 (and D500) focus system right up against some physical limits. Phase detect is angular sensitive. Restrict the angles at which the data is being obtained and you begin to make the system less accurate or eventually inoperative. Smaller apertures mean less angular differentiation. 

I’ve got a bunch of testing to do on this one, as I have to check this all against every autofocus area mode, too.  More later.

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