SLOG impact on Autofocus

[furfoot]

I'd really like to shoot in SLOG 2 on my Sony cameras (RX100 V, A7III) but have noticed that this picture profile has a negative impact on the autofocus performance. Especially in back-lit scenes the camera tends to focus on the light background rather than the foreground, I can imagine this being due to the lack of contrast in the image (I am using PDAF though). In any other Picture Profile the autofocus seems to work excellent.

Is this a known issue with regards to shooting in SLOG?

[thebrothersthre3]

Yes

[Deadcode]

40 minutes ago, thebrothersthre3 said:

Yes

Awesome, an expert! Please show examples, reason, and solution!

[thebrothersthre3]

15 minutes ago, Deadcode said:

Awesome, an expert! Please show examples, reason, and solution!

Auto focus doesn't always work, manual focus.

[Mark Romero 2]

7 hours ago, furfoot said:

I'd really like to shoot in SLOG 2 on my Sony cameras (RX100 V, A7III) but have noticed that this picture profile has a negative impact on the autofocus performance. Especially in back-lit scenes the camera tends to focus on the light background rather than the foreground, I can imagine this being due to the lack of contrast in the image (I am using PDAF though). In any other Picture Profile the autofocus seems to work excellent.

Is this a known issue with regards to shooting in SLOG?

I don't have an RX100 nor an a7 III, but autofocus works well when shooting in slog 2 on my a6500.

I often shoot in bad lighting and the only time I have noticed it glitch out is when I am temporarily panning across something like a solid color wall, and it usually just loses focus momentarily.

Otherwise, it seems to be fine. I have probably shot between about 500 to700 short (under 20 seconds long) clips in Slog 2 and I think I have only noticed it twice struggling in the conditions I mentioned above.

[Deadcode]

And none of you can figure out why this is happening. Beautiful.

The AF is reliable till -3EV (with A73), and live view exposure counts. 

Regarding to backlight situation if you go below -3EV the AF can struggle even in broad daylight.

The test is simple, use SLOG2, dial down to -2EV exposure and try to use AF. It will be unreliable.

If you dial up to +2EV everything will work perfectly.

In backlit situation the auto exposure metering is unreliable, if you push the exposure up manually above +2EV the AF will work perfectly.

Side note: in backlit situation i recommend to shoot with zebras turned on and set 46 +-5. Set your zebras to your subject, and you can avoid noise and still have some room in the highlights. (46IRE means +1EV for SLOG2 middle grey) If you are using auto metering, and set it to +2EV, it will be still underexposed. 

[Mako Sports]

Try to use GDA (gamma display assist) or an external monitor with a LUT to see critical focus.

[furfoot]

Yeah, I have been using Gamma Display Assist and I don't really have trouble with seeing if something is in focus or not. What I am seeing is that the autofocus system is being negatively affected by SLOG picture profiles, I was wondering whether this was due to the lack of contrast but as I'm using PDAF that didn't make sense to me. 

Deadcode's explanation is spot on, I had been mostly using auto metering in shutter priority with EV set to +2 (maximum allowed in video) which worked pretty well except for the afore mentioned heavy back lit scenario's. I will switch to manual and be more mindful of my exposure in various lighting situations to both aid the autofocus system and reduce shadow noise.