A7iii terrible AF with slog2

[nifty_five]

Howdy, y'all.  New here.  Hopefully a good place for info.

I have an A7iii.  I typically shoot video in HLG3, but yesterday I started playing with Slog2 for the first time in a while.  I ran a bunch of tests, and it appears that AF performance is terrible in Slog2, compared to HLG3.  Is this a known issue?  Anyone else experience this?

[Veres Deni Alex]

Hei there, welcome to the forum!

I honestly haven't been experiencing any issues with the autofocus when shooting S-Log, but I am curious to see what might be the problem.

I have no idea what is your style of shooting, and some screenshots of the footage would help to decide if different external factors contribute to this. I am not trying to question your experience, just trying to see what might be the problem, so I will propose a couple questions.

 

How do you expose? Autofocus will have to suffer if you don't overexpose by atleast +1 Stop.

Log gammas take 2 images simultaneously resulting into a high Dynamic Range image, but also a much, much darker image, so normally when people start shooting Log profiles they expose it exactly like other regular profiles, resulting in different problems, like underexposure which will definitely affect the auto focus.

Lighting direction also counts. Shooting against direct lighting will always raise problems in autofocus no matter camera. The a7III has a hybrid focus, which means it uses contrast based auto focus and phase detection to pinpoint the focus area.

The contrast based auto focus in combination with a Log profile can raise issues, because the picture profile is super de-saturated, extremely flat, so it basically presents very little contrast to work with, so it's going to be a difficult situation for the contrast based focus to pin point anything.

Also, one more curious detail which might or might not work, Can you try and let us know if turning on gamma assist is of any help and improves the autofocus? Normally the gamma assist is just a LUT, or a conversion to rec709 applied on top for viewing purposes, but which will not be baked in the footage, so this option might not improve your auto focus, but worth the try. 

 

Send over some screenshots or framegrabs so we can see what you are working with, that might help find a solution faster.

Thanks!

Edited May 4, 2023 by Veres Deni Alex

[nifty_five]

Thanks for the reply.  I tested with multiple scenes and lighting with plenty of contrast, using my face moving around in the frame as the subject.  Face tracking was on, wide area focus.

I exposed correctly, using zebras and the histogram.  In fact, I tried using zebras at 106+ AND using the at 93 +/-2 for skin.  The latter seemed to produce slightly better AF, but not much.  Gamma assist makes no difference, tried that too.

In every case, the camera focused MUCH more quickly and accurately in HLG3.

Edited May 4, 2023 by nifty_five

[Veres Deni Alex]

Exposing the skin at 93 on the zebra is extremely high, isn't it clipping? I normally would expose it at 50-60, depending on situation

Can you include a video to see a difference between the HLG and S-Log auto focus?

The extra processing needed for the log profile might affect the focus performance, I never noticed to be honest in my case, but that can also be the case.

 

[nifty_five]

93 works well in slog2, actually. Not clipped, nice ETTR.  I have a whole bunch of videos, I'll see what I can do.  If you have an A7iii, try it yourself.

[nifty_five]

Anyone else experience this?  It's quite obvious in my testing...

[Veres Deni Alex]

did not have the time for testing, but would love to see your results. can you share a link featuring both HLG and S-LOG footage autofocus?