I had an RX10M1, and had no problems with it. I bought an RX10M3 last July, primarily for the extra focal length, and while it is fine most of the time, there are times when I have had really hard times with the autofocus.
After testing, I came to the following conclusions:
In bright light, there are no problems with autofocus all the way up to 600mm. I do most of my shooting in Aperture Priority, with center focus. The pictures are crisp, even when cropped and magnified.
In anything less than bright light (shadows caused by a tree, or a cloudy day), it is very hard for the camera to autofocus above 200mm, and nearly impossible over 400.
When flying in a commercial plane over a city with a clean window and clear blue skies, focus is easy at 200mm, but the camera starts to encounter difficulty over 300mm, and is impossible over 500mm. Surprisingly, I have no such problems with my much cheaper Panasonic Lumix ZS 40!
When I try to focus on the edge of a shelf in bright light, the camera will auto-focus easily in portrait mode, but cannot focus in landscape mode. Likewise, if I try to focus on the vertical edge of a filing cabinet, the autofocus works in landscape mode, but not in portrait.
It won’t focus on the edge of a shadow..
Even when, in the last two points, it looks like the camera has focused, the focus light keeps blinking.
I have spoken to Sony about this, and they asked me to send the camera to Precision Camera. The first time, Precision Camera did a repair/replacement of the lens assembly, the main flex circuit and cpu. They said they did the general test, cleaned and repaired all functions and systems, and a cleaning. However, it did not solve the problems. When I sent it back, all that they did was to run the tests on it, and say it was working fine as it passed their tests. However, no one attempted to see if my problems had been resolved, which hadn’t been.
I’d like to have someone technical from Sony contact me, so that I can determine if this is expected behavior in the M3 (which would be very disappointing in a $1,600 camera), or whether there is something wrong with my unit. I want to be able to use the camera in its full focus range, and not just under special circumstances.
I look forward to conducting this discussion off-line.