How to lock autofocus

Question

Is there a way to prevent auto-focus from focusing, using any of the "standard" libraries, such as OpenCV, EmGU, DirectShow, etc?

I want auto-focus to find the optimal focus, then during video capture, prevent auto-focus from auto-focusing.

I know I can manually set the value, but that defeats the purpose of using auto-focus, and I have yet to find a way to obtain the optimal focus value as determined by the auto-focus.

Answer 1

For USB webcams that are UVC-compatible (as most are), there is a reasonable chance one can use the camera's autofocus and then lock it. To figure out whether the camera allows this via UVC, on Linux one can use v4l2-ctl, which is in package v4l-utils. v4l2-ctl -l lists all available controls, v4l2-ctl -c sets the value for a control, and v4l2-ctl -C gets the value.

For example, the following commands did the trick for a Microsoft LifeCam Cinema on an Ubuntu 16.04 box that had a simple Python OpenCV program running to display the current frame:

> v4l2-ctl -d 0 -c focus_auto=1
> v4l2-ctl -d 0 -C focus_absolute
focus_absolute: 12

After moving the object closer to the camera, the focus changed, and I got a different value for focus_absolute: (So UVC gives access to what what value the autofocus picked.)

> v4l2-ctl -d 0 -C focus_absolute
focus_absolute: 17

I then changed to manual focus and this locked the value the autofocus had picked:

> v4l2-ctl -d 0 -c focus_auto=0
> v4l2-ctl -d 0 -C focus_absolute
focus_absolute: 17

So for the LifeCam Cinema, the only thing the code would need to do is change the focus_auto control initially to auto (1) and then to manual once the focus is to be locked.

From Python, I typically run v4l2-ctl simply by using subprocess.check_output(). I recall seeing Windows libraries for UVC, but never played with them.