MAC OS X: Set external USB camera resolution with AVCaptureSession's sessionPreset doesn't work

Question

I am trying to set the camera's resolution for the capture session using this:

[session setSessionPreset:AVCaptureSessionPresetLow];  // or any others resolutions

It doesn't seem to make any difference. It always uses the highest resolution the camera can offer. This is a UVC compliant USB camera and wasn't made by Apple.

Question: Can setSessionPreset apply to external USB non-Apple camera? Or my problem is something else?

Edit: I also tried this test:

if ([session canSetSessionPreset:AVCaptureSessionPresetLow])
{
    NSLog(@"can presetlow");    // this part got called.
    [session beginConfiguration];
    session.sessionPreset = AVCaptureSessionPresetLow;
    [session commitConfiguration];
}
else
{
    NSLog(@"no, can't presetlow");
}

Bounty question: How to set the camera resolution in this case?

Answer 1

It may not be possible, depending on the camera. UVC compatible does not mean that all features are supported.

http://www.mac-compatible-web-cam.com/?ID=McRmr2

A work-around, depending on your application, might be to take the high-res image and convert it to lower res in your code.