This is a continuation of a series of posts on the Nikon D850. The series starts here. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series; just look for “D850”.
A while back I wrote this post about the accuracy of manual focusing with the D850 using the “focus confirmation” feature, aka the green dot (it’s not green in the D850), aka “the meatball”.
A two word summary of the accuracy and repeatability of this method: not very.
In researching the presence or absence of D850 PDAF compensation for focus shift, several people introduced me to an AF tuning procedure that uses the meatball. I tried it, and initially, it didn’t work for me; I focused manually and saw the meatball all lit up with the AF focus trim set from -20 to +20. I turns out the Nikon engineers have a (insert pejorative adjective here) trick up their collective sleeves: in manual focus mode, the meatball tolerance is deliberately widened so that it’s easier to get it to light up.
For people who care about manual focusing accuracy, this is a disaster, and it explains part of the rotten results I got when I tried to use it under controlled conditions.
But there’s a workaround. It has been known to the Dot Tune inventor for quite some time but is new to me. It may be new to you, too, and it has implications that extend far beyond Dot-Tune users. It affects everyone trying to us the meatball for accurate manual focus.
Here’s the trick:
- Set the camera in AF mode.
- Set the lens in AF mode.
- Set the camera up for back-button focusing only; defeat the option that allows the camera to focus with a half-press of the shutter release.
- Half-press the shutter release to see the arrows and the meatball.
- Focus until the arrows go out and only the meatball remains.
- Press the shutter release all the way down.
With the 105 mm f/1.4 lens, this makes the meatball tolerance — the range where the meatball will stay on — a bit less than 10 AF trim units. That’s not very low, but it’s a heck of a lot lower than the more than 40 AF trim units when the camera is in MF mode. You’ll still have to deal with the vagaries of the phase detection mechanism, and there seems to be no correction for focus shift, but you’ll be a lot better off than using the meatball in MF mode.
It’s hard to say for sure what the camera does when you affix a lens that doesn’t support autofocus, but it appears to me that the camera switches to the loose-tolerance mode for the meatball in that case.
I still say, with the D850, stick with live view if you want accurate manual focusing.