D850 focusing errors when using IR AF assist

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Seashores New Member • Posts: 9
D850 focusing errors when using IR AF assist

Hi,

I encountered an odd problem, but before I begin I will emphasize: all my lenses are carefully calibrated on both my D850 bodies and in normal circumstances AF works admirably.

Problem arises when I'm using D850 with an SB-5000 in AF-S mode with IR AF assist from the speedlight. Issue is that focus consistently falls behind the desired subject, as if the lens isn't properly calibrated.

However, when I either turn off the IR assist or use AF-C (which doesn't use it), focus is performing correctly. It appears as if the IR assist is the source of confusion.

To make things even more peculiar, this happens on both D850 bodies I own and it happens while using Nikon 24/1.4 and Sigma 35/1.4 Art, but it does not affect Nikon 58/1.4.

I first noticed this with the Sigma lens, so naturally I though that since IR light focuses differently than visible light, Nikon lenses may use additional data to correct for this. Noticing this on Nikon 24/1.4 disproved this theory. Then I thought maybe it's a systemic error on all the lenses, but confusingly, it didn't affect 58/1.4 which should be even more sensitive to AF issues. Finally, I noticed it affects two different D850s with two different SB-5000.

Currently, I'm at a loss and have no more theories.

D850 has amazing AF, so when I shoot events at night I often just use AF-C and keep moving, so I never noticed this before. But I recently encountered such dark conditions that I had to fall back to AF-S and IR assist, which frustratingly produced pretty consistently blurry images (focus behind the subject), with a few exceptions (maybe 30% were in focus).

Any ideas on what is causing this or how to solve it? Is there anyone with the same equipment here that is able to reproduce this issue?

Thanks!

Nikon D850
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