title: AVCaptureSession resolution doesn't change with AVCaptureSessionPreset
body: <p>I want to change the resolution of pictures I take with the camera on OS X with AV Foundation.</p>
  <p>But even if I change the resolution of my AVCaptureSession, the output picture size doesn't change. I always have a 1280x720 picture.</p>
  <p>I want a lower resolution because I use these pictures in a real time process and I want the program to be faster.</p>
  <p>This is a sample of my code:</p>
  <pre><code> session = [[AVCaptureSession alloc] init];
  
  if([session canSetSessionPreset:AVCaptureSessionPreset640x360]) {
      [session setSessionPreset:AVCaptureSessionPreset640x360];
  }
  
  AVCaptureDeviceInput *device_input = [[AVCaptureDeviceInput alloc] initWithDevice:
                                         [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo][0] error:nil];
  
  if([session canAddInput:device_input])
      [session addInput:device_input];
  
  still_image = [[AVCaptureStillImageOutput alloc] init];
  
  NSDictionary *output_settings = [[NSDictionary alloc] initWithObjectsAndKeys:AVVideoCodecJPEG, AVVideoCodecKey, nil];
  [still_image setOutputSettings : output_settings];
  
  [session addOutput:still_image];
  </code></pre>
  <p>What should I change in my code?</p>