iPhone image ratio captured from AVCaptureSession

I am using somebody's source code for capturing image with AVCaptureSession. However,I found that CaptureSessionManager's previewLayer is shotter then the final captured image.

I found that the resulted image is always with ratio 720x1280=9:16. Now I want to crop the resulted image to an UIImage with ratio 320:480 so that it will only capture the portion visible in previewLayer. Any Idea? Thanks a lot.

Relevant Questions in stackoverflow(NO good answer yet): Q1, Q2

Source Code:

- (id)init {
if ((self = [super init])) {
    [self setCaptureSession:[[[AVCaptureSession alloc] init] autorelease]];
}
return self;
}

- (void)addVideoPreviewLayer {
[self setPreviewLayer:[[[AVCaptureVideoPreviewLayer alloc] initWithSession:[self captureSession]] autorelease]];
[[self previewLayer] setVideoGravity:AVLayerVideoGravityResizeAspectFill];

}

- (void)addVideoInput {
AVCaptureDevice *videoDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];   
if (videoDevice) {
    NSError *error;


    if ([videoDevice isFocusModeSupported:AVCaptureFocusModeContinuousAutoFocus] && [videoDevice lockForConfiguration:&error]) {
        [videoDevice setFocusMode:AVCaptureFocusModeContinuousAutoFocus];
        [videoDevice unlockForConfiguration];
    }        

    AVCaptureDeviceInput *videoIn = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:&error];
    if (!error) {
        if ([[self captureSession] canAddInput:videoIn])
            [[self captureSession] addInput:videoIn];
        else
            NSLog(@"Couldn't add video input");     
    }
    else
        NSLog(@"Couldn't create video i