IOS captureStillImageAsynchronouslyFromConnection: completeHandler: dark image

Question

I use captureStillImageAsynchronouslyFromConnection:completionHandler:to capture photos in my application. Unfortunately, when a user takes a picture right after viewing, the image is very dark, for iPhone 4 and iPhone 5 almost black.

As suggested in other SO questions / answers, I implemented KVO to adjust exposure, focus, and white balance and waited for them to stop. But still after the implementation of this image is still dark when they are recorded immediately after setting up the session.

Any ideas?

I use sessionPreset = AVCaptureSessionPresetPhoto