This is a continuation of a series of posts on the Nikon D850. 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”. This is equally a post about the Sony a7RIII and is also tagged as such.
In the last two posts, I summarized my thoughts on first, the D850 and next, the a7RIII autofocus accuracy. Now I’ll compare the systems on the two cameras. My conclusions may be affected by my choice of lenses. I tend to use the D850 for big, heavy, fast lenses, and the a7RIII for slower, lighter ones.
However, I did test two lenses that are entirely comparable: the Nikon 80-400 VR II on the D850 and the Sony 100-400 on the a7RIII. Sometimes the combination of outstanding Sony lenses and the tuning-free autofocus system (dotted lines below) can deliver results that put the D850 (solid lines) to shame:
Heavy lines are the means. Light ones are means plus and minus one standard deviation. By convention, front-focusing is plotted as negative CoC diameters, and back-focusing as positive ones. The low spread of the a7RIII is due to my choosing the large spot size for the above comparison. For reference with respect to the circle of confusion (CoC) diameters, the pixel pitch of the two cameras is a bit under 5 micrometers (um).
The Sony combination above even looks very good with medium spot size, which sometimes shows more variation:
In AF-C mode, the a7RIII has more variation, but the D850 does not:
The a7RIII does not compensate for focus shift in AF-S mode, but at this focal length and distance (about 7 meters) the 100-400 has so little focus shift that that’s not an issue, and you’re better off with AF-S on the a7RIII.
If we compare the 58/1.4 on the Nikon to the Zony 55 on the Sony, with face detection for the D850 and eye detection for the a7RIII:
The D850 shows commendably low variation in face detection mode.
There’s a pattern here that holds most of the time: when precisely tuned for the focal length and distance (as in the above plots), and also for the taking aperture (as was not done in the above plots; the tuning was done wide open), and the D850 uses face detection (and probably some of the other wide modes) the D850 can hold its own against the a7RIII. But start taking away those provisos, and the D850 starts to flag.
There’s another time when the D850 can look pretty darned spectacular: pair it with a lens with little focus shift and low LoCA in either PDAF (dotted below) or CDAF (solid below):
Although Nikon does well in some of the prelims, the Sony wins the autofocus accuracy contest by the end of the competition.