Adjust the hardness of the blur effect in iOS 8

I am using the blur effect on an image view in iOS 8 and I wonder if it's possible to tell swift how strong the blur effect should be.

This is the code..

    var blurEffect = UIBlurEffect(style: UIBlurEffectStyle.Dark)
    var visualEffectView = UIVisualEffectView(effect: blurEffect) as UIVisualEffectView
    visualEffectView.frame = imageView.bounds

    imageView.addSubview(visualEffectView)