Lumix S5ii video AF still (a bit) contrast based?

[bolandross]

Hey folks,

I noticed something strange when using my new Lumix S5ii: When recording video (4k 24p) with continuous auto focus enabled, the camera seems to have problems focusing objects that have a plain color / no visual pattern. As soon as something more "contrasty" is in the focus box, AF becomes confident again.

This is something I know from using contrast base focus cameras, shouldn't this be "fixed" with the new S5ii?

 

[pdk42]

PDAF still needs some subject contrast. My understanding is that it's analogous to a rangefinder. In a traditional DSLR, there is an optical splitter that splits the incoming light (left/right, or up/down, or even both) into two views. By comparing the amount of split (the phase difference), it knows how far out of focus it is. But it still needs some image "texture" to do the comparison. If you pointed the lens to a blank wall, the two split images would look the same whether they were in focus or not.

Now, on-sensor PDAF is a more complex trick, but the principle is the same. In this case there is no single optical splitter, but each AF pixel is effectively doing its own split and the camera's AF firmware uses pixels separated across the sensor to determine the phase difference.

Finally, it's not uncommon for AF systems to use a combination of PDAF and CDAF. The heavy-lifting is done with PDAF but a little "shimmy" is often done with CDAF to get the best accuracy.

Exactly how the S5ii (or any other camera) works is hard to determine because the manufacturers believe we are all numbskulls and refuse to tell us the details. All we get is marketing guff!

 

[bolandross]

Thank you so much @pdk42 for this helpful explanation. It seems like I always had a wrong impression of PDAF works, thinking of "invisible laser beam dots" that are sent from the camera to the front and then some DSP checks how these dots look like (bigger or smaller) and calculating the distance to the object that should be focused. Happy to have learnt something new!

 

[bolandross]

and in defense of Panasonic, they call it »phase hybrid autofocus« after all

 

[pdk42]

Thank you so much @pdk42 for this helpful explanation. It seems like I always had a wrong impression of PDAF works, thinking of "invisible laser beam dots" that are sent from the camera to the front and then some DSP checks how these dots look like (bigger or smaller) and calculating the distance to the object that should be focused. Happy to have learnt something new!

Yes, all current camera systems use passive techniques (just looking at the scene). Some of the early AF systems used ultra-sound as a sort of "sonar" ranging system, but they didn't pass the test of time!

 

[Threlly]

I believe the AF in the S5II will use contrast where and when it feels like it, although my video tests so far have shown it to be the equal to the Sony bodies I've had in the last 5 years.
The body/face tracking seems to work well even in low light, but I'm mostly a manual guy myself.

It would be interesting if Panasonic explained how their AF algorithm worked in respect of contrast but it's seemingly unlikely that they would go that far, IP being what it is.