This is a continuation of a series of posts on the Sony a7RIII. 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 “a7RIII”.
Yesterday I summarized my findings over the last two months relating to the autofocus accuracy of the D850. Now I’ll do the same for the a7RIII. In a future post, I’ll compare and contrast (shades of high school!) the two cameras’ autofocus prowess.
The D850 does phase-detection autofocus (PDAF) in the finder and contrast-detection autofocus (CDAF) using the sensor. The user gets to control which system is employed. The a7RIII is more flexible by design, and more opaque in its implementation. It, too, has both CDAF and PDAF, but both systems are sensor-based, since there is no finder, ground glass, nor flipping mirror. Sony doesn’t talk much about how the two systems are arrayed in battle against defocus blur, but the camera appears to employ both depending on the settings, the lens attached, and even, in the case of the a7RII, the firmware revision number. I’m not going to enter the thicket of firmware revisions, but just describe how the a7RIII with firmware 1.00 works.
The combination of Sony’s reticence in discussing how their cameras autofocus, the existence of two modes, and the change in behavior with different lenses have sent their customers into paroxysms of arcane reverse engineering to figure out just what the heck is going on, and wrong answers abound. I myself, before I developed the tools that I’ve been using for the past month, was guilty of spreading what have turned out to be falsehoods, and I am happy to be able to undo the major one here.
In testing the a7RIII with the following lenses:
- Sony/Zeiss 55 mm f/1.8 (Zony 55)
- Sony 100-400 mm
- Sony 90 mm f/2.8 macro
- Zeiss Batis 85 mm f/1.8
- Sony 70-200 mm f/4
In AF-S mode, the a7RIII focuses with the lens diaphragm wide open, or very close to that. That means that the images are prone to suffer from focus shift. Some of the above lenses have remarkably little focus shift, so the way the a7RIII works is not always problematical, but others have significant focus shift.
Here’s a summary of the worst-case focus shift effects in size of the circles of confusion that result in micrometers:
- Sony/Zeiss 55 mm f/1.8 (Zony 55) — 2 um (but that gets worse using AF-S)
- Sony 100-400 mm at 400 mm — 4 um
- Sony 100-400 mm at 100 mm — 2 um
- Sony 90 mm f/2.8 macro — 5 um
- Zeiss Batis 85 mm f/1.8 — 10 um
- Sony 70-200 mm f/4 at 100 mm — 4 um (but AF-S makes that even worse)
I should note that there is far less focus shift for these lenses than many of the Nikon ones that I tested on the D850.
I don’t know of a workaround for this AF-S behavior other than using AF-C, which works because, in AF-C mode, the a7RIII focuses with the lens diaphragm at taking aperture, and focus shift is not a problem. It is ironic to me that the D850, supposedly the exemplar of obsolete tech in modern clothes, offers the user total control over which mode is used, and the wave-of-the-future a7RIII does not.
Some people have said that there are lenses that I haven’t tested that focus at taking aperture in AF-S mode. That is entirely possible, but there are lenses listed above that people said focus that way, so I’m from Missouri on this one, at least for now.
So what’s wrong with setting the camera to AF-C almost all the time? AF-C gives more focus variation than AF-S.
Using face detection can reduce focus plane variability.
Large spot sizes tend to create less variability in the focus plane than small or medium ones. That’s probably why face detection also helps, but that doesn’t explain the next one.
Using eye detection can reduce focus plane variability.
There is conventional wisdom that stopping down in AF-C increases variability. That is not always the case. This is about the worst AF-C variability I’ve seen at wide f-stops: