I have previously solved a camera overlay issue: like this
CGSize screenBounds = [UIScreen mainScreen].bounds.size;
CGFloat cameraAspectRatio = 4.0f/3.0f;
In iOS 8 you no longer need to scale if you want to fill the screen. So the unique thing you have to do is to comment the assignment:
//self.picker.cameraViewTransform
That is to say, self.picker.cameraViewTransform = CGAffineTransformMakeScale(1.0, 1.0) conforms to self.picker.view.frame automatically.
In conclusion, the work is already done for you by UIImagePickerController.