Matlab return usb webcam focus values not consistency

Hi, I am using the webcam C930e.

I can return the value of the focus property using the following three lines:

cam = webcam(id);

cam.FocusMode = 'auto';

af=cam.Focus.

However, for the same controlled environment the webcam is viewing, in some occassion the returned af=0, and in most occassion af=30. I wish to find a way to troubleshoot and understand this changes of the af value. Please explain what does the af=0 or other value means. Does af=0 means the webcam is not in autofocus more or ? and if there is a way I can troubleshoot why occassional af value changes, how do I get around not having the af to change for the same environment that the webcam is viewing? Thanks.

[Walter Roberson]

I am not finding much about the C930e. I am finding a number of YouTube about working with autofocus on the C920, with people taking the camera apart or using external lens on it.

FocusMode does not show up for my C920 on the Mac; I will experiment to see if it shows up on Windows.

[Walter Roberson]

I was able to set FocusMode under windows. However, I was running MATLAB in a Virtual Machine hosted by my Mac, and under that chain of emulations, it was not able to access the frame buffer of the USB webcam.