Switching AVCaptureSession preset when capturing a photo

My current setup is as follows (based on the ColorTrackingCamera project from Brad Larson):

I'm using a AVCaptureSession set to AVCaptureSessionPreset640x480 for which I let the output run through an OpenGL scene as a texture. This texture is then manipulated by a fragment shader.

I'm in need of this "lower quality" preset because I want to preserve a high framerate when the user is previewing. I then want to switch to a higher quality output when the user captures a still photo.

First I thought I could change the sessionPreset on the AVCaptureSession but this forces the camera to refocus which break usability.

[captureSession beginConfiguration];
captureSession.sessionPreset = AVCaptureSessionPresetPhoto;
[captureSession commitConfiguration];

Currently I'm trying to add a second AVCaptureStillImageOutput to the AVCaptureSession but I'm getting an empty pixelbuffer, so I think I'm kinda stuck.

Here's my session setup code:

...

// Add the video frame output
[captureSession beginConfiguration];

videoOutput = [[AVCaptureVideoDataOutput alloc] init];
[videoOutput setAlwaysDiscardsLateVideoFrames:YES];
[videoOutput setVideoSettings:[NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA] forKey:(id)kCVPixelBufferPixelFormatTypeKey]];
[videoOutput setSampleBufferDelegate:self queue:dispatch_get_main_queue()];

if ([captureSession canAddOutput:videoOutput])
{
    [captureSession addOutput:videoOutput];
}
else
{
    NSLog(@"Couldn't add video output");
}

[captureSession commitConfiguration];



// Add still output
[captureSession beginConfiguration];
stillOutput = [[AVCaptureStillImageOutput alloc] init];

if([captureSession canAddOutput:stillOutput])
{
    [captureSession addOutput:stillOutput];
}
else
{
    NS