AVCaptureVideoPreviewLayer looks different than captured Image - ios

Question

The image presented in my PreviewLayer has decent color/light levels, but when I capture the image, and put it in a UIImageView it looks super dark.
Why are the light levels so different?
And here's the controller code
//Parent Class
class CameraViewController: UIViewController {
#IBOutlet var photoPreviewView: UIView!
let captureSession = AVCaptureSession()
var previewLayer: AVCaptureVideoPreviewLayer?
var captureDevice: AVCaptureDevice?
var imageData: NSData?
override func viewDidLoad() {
super.viewDidLoad()
captureSession.sessionPreset = AVCaptureSessionPresetMedium
let devices = AVCaptureDevice.devices()
for device in devices {
// Make sure this particular device supports video
if (device.hasMediaType(AVMediaTypeVideo)) {
// Finally check the position and confirm we've got the back camera
if(device.position == AVCaptureDevicePosition.Back) {
captureDevice = device as? AVCaptureDevice
}
}
}
if captureDevice != nil {
beginSession()
}
}
func captureImage(callback:(NSData)->()) {
let stillImageOutput = AVCaptureStillImageOutput()
captureSession.addOutput(stillImageOutput)
let videoConnection = stillImageOutput.connectionWithMediaType(AVMediaTypeVideo)
if videoConnection != nil {
stillImageOutput.captureStillImageAsynchronouslyFromConnection(videoConnection) { (imageDataSampleBuffer, error) -> Void in
callback(AVCaptureStillImageOutput.jpegStillImageNSDataRepresentation(imageDataSampleBuffer))
}
} else {
println("couldn't find video connection")
}
}
func beginSession() {
var err : NSError? = nil
let captureDeviceInput = AVCaptureDeviceInput(device: captureDevice, error: &err)
captureSession.addInput(captureDeviceInput)
if err != nil {
println("error: \(err?.localizedDescription)")
}
let previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
photoPreviewView.layer.addSublayer(previewLayer)
//FIXME Preview layer is not being positioned as expected. This is an arbitrary hack to make it "look right" on my iphone6
previewLayer.frame = CGRect(x: -64, y: 0, width: 504, height: 504)
captureSession.startRunning()
}
}
class SubmitGuessViewController: CameraViewController {
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!) -> Void {
super.prepareForSegue(segue, sender: sender)
let createImageFromData = {(imageData: NSData) -> () in
if ( imageData.length > 0 ) {
let checkGuessViewController = segue.destinationViewController as CheckGuessViewController
checkGuessViewController.submittedImage = UIImage(data: imageData)
} else {
println("Image Data not captured")
}
}
self.captureImage(createImageFromData)
}
}