The other day I had an ‘oh no!’ moment involving the autofocus system on my Nikon Z6 fitted with a 70-200mm zoom.

Most of the time the autofocus with my Z-series cameras works very well. On rare occasion it misses completely.

I was set-up at Christiana, Pennsylvania along Amtrak’s former Pennsylvania Railroad Main Line waiting for westward Keystone service number 605 in order to catch it passing the old PRR station.

I’d set the autofocus to ‘single-point’ (which allows to preselect a distinct point in the frame as the desired place of sharpness) and the system to ‘AF-C’ (continuous), a mode that in theory should continuously adjust the focus on the subject point.

There were three complicating conditions that in combination yielded an undesirable result. 1) The scene was back-lit with bright morning sun, which can make it more difficult for the autofocus system to quickly pick the focus on the desired point. 2) The train was moving faster than 90mph, which not only made it difficult to focus, but gave me no room for error when the shutter was released. 3) The headlights on Amtrak’s ACS-64 use a form of LEDs that produce a wavelength that can momentarily confuse the autofocus system on the camera. I’ve experienced these unfortunate effects previously.

The result was one photo where the focus was slightly off, followed by a second closer image where the focus was missed completely.

One solution for future efforts; I can use the autofocus to pre-focus on the desired location and the switch it off, thus avoiding the condition where at the last split second the focus shifts. But this too is a gamble, and doesn’t always work as hoped.

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