How to set size for AVAssetImageGenerator?

I try to read some frames from a video. The video is 640 x 480 but the images I get are only 480 x 360. Is there a way to get images of the original size?

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info 
{
    NSString * mediaType = [info objectForKey:UIImagePickerControllerMediaType];
    if ([mediaType isEqualToString:(NSString *)kUTTypeMovie])
        [self readMovieFrames:[info objectForKey:UIImagePickerControllerMediaURL]];

    [self dismissViewControllerAnimated:YES completion:nil];
}

- (void)readMovieFrames:(NSURL *)url
{
    AVPlayerItem *playerItem = [AVPlayerItem playerItemWithURL:url];
    AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc] initWithAsset:playerItem.asset];
    imageGenerator.requestedTimeToleranceAfter = kCMTimeZero;
    imageGenerator.requestedTimeToleranceBefore = kCMTimeZero;
    imageGenerator.appliesPreferredTrackTransform = YES;   
    //…
    CGImageRef imageRef = [imageGenerator copyCGImageAtTime:requestTime actualTime:&actualTime error:&error];
    UIImage *img = [UIImage imageWithCGImage:imageRef];
    UIImageWriteToSavedPhotosAlbum(img, nil, nil, nil);
    CGImageRelease(imageRef);  
}

Either use

imageGenerator.maximumSize = CGSizeZero; 

to get an unscaled image or define a bounding box of 640px, aspect ratio remains

imageGenerator.maximumSize = CGSizeMake(640, 640);

Hope that helps!