Sony A9III + Sony 100-400 GM big problem with AF

[Marcin Jasinski]

Hello, I’m having an issue with the A9III when paired with the Sony 100-400mm f/4.5-5.6 GM lens. In AF-S mode, everything works as it should. However, in AF-C mode during a football match, I experienced a problem.

When I try to shift focus from one subject, like a player, to something further away like the stands—so making a significant change in focusing distance—it can take up to a second to refocus. I press the shutter halfway and sometimes have to wait almost a full second.

When the subjects are relatively close to each other in distance, there's no major issue. But when I need to quickly refocus on someone much closer, the camera seems to pause for about a second before starting to focus. I disabled Pre-Capture, and it doesn’t make a difference whether AF tracking is on or off—it behaves the same. I also disabled subject recognition (people), changed the AF speed settings, but nothing helps. This behavior only occurs in AF-C mode with this lens. It doesn’t happen all the time, but in most cases it does.

Now, perhaps the most important part: I noticed that if I just only tap very fast the focus button, the lens/camera focuses instantly. But if I press and hold it (to track or maintain focus on a subject), it takes significantly longer. In AF-S mode, it also works perfectly.

With other lenses, like the 70-200 GM2, everything works great. On the A7RIII, this 100-400mm lens also works without issues. Both the camera and lens firmware are up to date.

It is like camera freeze for a second and after that it focus.

I tested on 6 my other lenses and everything is ok. 

Summary:

• The problem occurs only in AF-C mode.
• AF-S mode works flawlessly.
• The issue mainly appears when making large focus distance changes (e.g., from the stands to a player).
• A quick AF button press results in instant focus, but holding it causes noticeable delay.
• Other lenses (e.g., 70-200 GM2) work perfectly on the A9 III.
• The 100-400 GM performs without issues on the A7RIII.
• All firmware is up to date.
• Disabling Pre-Capture, face detection, and other settings does not resolve the issue.