iOS - access back camera error

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Device : Iphone 5 IOS 6.0 XCode : 4.6 OpenCV lib: 2.4.3

I went through the tutorial, it worked fine in IOS6. However, when I want to switch to camera device on back (it is on front in tutorial), I modified the code:

self.videoCamera.defaultAVCaptureDevicePosition = AVCaptureDevicePositionFront;

to:

self.videoCamera.defaultAVCaptureDevicePosition = AVCaptureDevicePositionBack;

However, there is nothing happened. is this a bug in OpenCV 2.4.3?

Besides, is OpenCV compatible with IOS 5.1 or lower versions? When I ran it in IOS 5.1, there is exceptions:

'NSInvalidUnarchiveOperationException', reason: 
           'Could not instantiate class named NSLayoutConstraint'