IPhone How to set frame rate and slower AVCapture didOutputSampleBuffer Delegate

I want to slow down the frame rate of a video device on an iPhone 4S so that the didOutputSampleBuffer delegate is called less frequently. This should improve performance as I process every frame and need a large frame for detail.

I tried using the following to do it when I set up the AVSession:

AVCaptureConnection *conn = [self.output connectionWithMediaType:AVMediaTypeVideo];
[conn setVideoMinFrameDuration:CMTimeMake(1, CAPTURE_FRAMES_PER_SECOND)];
[conn setVideoMaxFrameDuration:CMTimeMake(1, CAPTURE_FRAMES_PER_SECOND)];

      

        
        
        
      

    

But it has no effect, I can change the CAPTURE_FRAMES_PER_SECOND from 1 to 60 and not see the difference in performance or slowing down the video capture. Why doesn't it have any effect? How can I slow down the capture frame rate for a video device?

I established my session with the following code:

// Define the devices and the session and the settings
self.session = [[AVCaptureSession alloc] init];

//self.session.sessionPreset = AVCaptureSessionPresetPhoto;
//self.session.sessionPreset = AVCaptureSessionPresetHigh;
self.session.sessionPreset = AVCaptureSessionPreset1280x720;

self.device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
self.input = [AVCaptureDeviceInput deviceInputWithDevice:self.device error:nil];

// Add the video frame output
self.output = [[AVCaptureVideoDataOutput alloc] init];
[self.output setAlwaysDiscardsLateVideoFrames:YES];
self.output.videoSettings = [NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA]
                                                        forKey:(id)kCVPixelBufferPixelFormatTypeKey];

// A dispatch queue to get frames
dispatch_queue_t queue;
queue = dispatch_queue_create("frame_queue", NULL);

// Setup the frame rate    
AVCaptureConnection *conn = [self.output connectionWithMediaType:AVMediaTypeVideo];
[conn setVideoMinFrameDuration:CMTimeMake(1, CAPTURE_FRAMES_PER_SECOND)];
[conn setVideoMaxFrameDuration:CMTimeMake(1, CAPTURE_FRAMES_PER_SECOND)];

// Setup input and output and set the delegate to self
[self.output setSampleBufferDelegate:self queue:queue];
[self.session addInput:self.input];
[self.session addOutput:self.output];

// Start the session
[self.session startRunning];

      

        
        
        
      

    

I am grabbing frames using the "didOutputSampleBuffer" delegate below:

// The delegate method where we get our image data frames from
- (void)captureOutput:(AVCaptureOutput *)captureOutput
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
       fromConnection:(AVCaptureConnection *)connection
{

    // Extract a UImage
    CVPixelBufferRef pixel_buffer = CMSampleBufferGetImageBuffer(sampleBuffer);
    CIImage *ciImage = [CIImage imageWithCVPixelBuffer:pixel_buffer];

    // Capture the image
    CGImageRef ref = [self.context createCGImage:ciImage fromRect:ciImage.extent];

    // This sets the captured image orientation correctly
    UIImage *image = [UIImage imageWithCGImage:ref scale:1.0 orientation:UIImageOrientationLeft];

    // Release the CGImage
    CGImageRelease(ref);

    // Update the UI on the main thread but throttle the processing
    [self performSelectorOnMainThread:@selector(updateUIWithCapturedImageAndProcessWithImage:) withObject:image waitUntilDone:YES];

}