private float minScale = 1f;
private float doubleTapScale = 2f;
private float maxScale = 4f;

private void SetUpDoubleTapZoom()
{
    imageViewToZoom.ContentMode = UIViewContentMode.ScaleAspectFit;
    scrollView.MaximumZoomScale = maxScale;
    scrollView.MinimumZoomScale = minScale;

    var doubleTap = new UITapGestureRecognizer(OnDoubleTap)
    {
        NumberOfTapsRequired = 2
    };

    scrollView.AddGestureRecognizer(doubleTap);
}

private void OnDoubleTap(UIGestureRecognizer gesture)
{
    scrollView.ZoomScale = (scrollView.ZoomScale.Equals(minScale)) ? doubleTapScale : minScale;
}

private float minScale = 1f;
private float maxScale = 4f;

private void SetUpPinchGestureZoom()
{
    imageViewToZoom.ContentMode = UIViewContentMode.ScaleAspectFit;

    scrollView.MaximumZoomScale = maxScale;
    scrollView.MinimumZoomScale = minScale;

    scrollView.ViewForZoomingInScrollView += (UIScrollView sv) => { return imageViewToZoom; };
}