title: How to force AVCaptureDevice to auto-adjust exposure
body: <p>Is there any way to force an <code>AVCaptureDevice</code> on the Mac to adjust its exposure? I'm using the camera to take a photo, and boosting the screen's brightness to throw as much light as possible, but this is causing the exposure to get blown out. I'm setting its <code>exposureMode</code> to Continuous Auto Exposure, but it doesn't look like that responds as quickly as I'd like.</p>
  
  <p>Is there a way to call <code>[device adjustExposure]</code> or something like that? I'm <strong>not</strong> looking to manually adjust the exposure, which would be overkill here.</p>