Panasonic G7 Camera focus problem with example

alcelc wrote:

... When looking at this video clip frame by frame I noticed that I had zoomed out a bit at the start of this video clip and it did not effect the focus.

I suppose it would be bet on our luck ? Yes, usually our cameras behalf better at the beginning.

For some unknown reason the focus seems to move to a focus that would be used when video recording something close to the camera rather than put the subject slightly out of focus. I'm wondering if the camera sends a constant signal to the lens to keep the camera correctly focused and if it lost this signal then the lens with go to closeup focus. There is also a delay when the camera has lost focus as most of the time the focus is very fast. I'm wondering why it takes so long before it attempts to refocus.

Zooming out slightly should not change the contrast in the center of the screen that is needed for accurate focusing. I can understand if the girl moved forwards and the camera then focused on the background but this is not the case as there is no attempt to focus on the background. I see no reason why the camera should go completely out of focus and not slightly out of focus.

Brian

It has been well discussed over internet and I suppose you can find a lot of reports on this. A recent test I watched : https://www.youtube.com/watch?v=NvkrWGjWY9k comparing various latest Pany models (can skip to 2:01).

I have no doubt that you must be well aware how CDAF work. CDAF in principal has to look for a focus on the entire sensor surface, so reduce the number of its focus point (sector) and restrict the area (size of the focus box) to look for a focus can shorten the time, i.e. reduce AF hunting.

When doing AF tracking (continuous AF in video), basically it uses all of the usable sensor area searching for a possible focus point. It starts to show its disadvantage vs PDAF which has only to watch on certain focus points and so in theory shorter time be needed. Because of the fixed line up of the PDAF points, it also allows the camera to predict the direction of movement easier. CDAF of, e.g. my GX85, has to first look at the 49 areas, and from the likely area find the highest contrast point and focus on it... Can imagine how inefficient it is (relative to PDAF).

Back to G7, the camera under Continuous AF will keep on searching for the sharpest focus. If the object movement is small (it might hunt even on still object, just smaller chance only), we can usually have a more stable focused result. When we zoom in or out, the camera will check for the focus again and hunting will appear. DFD theoretically can reduce the period of hunting. AF hunting generally cannot be avoided, just a matter of how fast it will return sharp again.

I think improved AF algorithm (GH5, G9 etc?), more powerful processing power, or use On Sensor PDAF (as Sony, Canon, Fuji and EM1-II) in future might keep AF hunting minimal.

Thanks alcelc for the info ans video link.

In your reply you said "When we zoom in or out, the camera will check for the focus again and hunting will appear" This does not always happen. I posted a second short video to YouTube you can find it at this address. The video shows zooming and no refocusing.

https://youtu.be/ObFDFeReuPw

As you can see from the video when zooming it does not always adjust the focus after the zooming has stopped. This makes me wonder why sometimes that focus is checked and not at other times.

I have the camera set to focus on what ever is in the middle of the frame spot but maybe with continuous AF this gets overridden and the camera uses a different focus method.

You wrote about DFD, what is that?

Some say professionals never zoom but if your panning someone walking along the road, coming towards you then need to pan to prevent the person from being too big to fit into the frame.

Brian