<ScrollViewer Name="ScrW">
     <ViewBox Name="ViewB" Stretch="UniformToFill">
          <Canvas Name="Field" ClipToBounds="True">
               <!--Некая графика, превышающая размер экрана-->
          </Canvas>
     </ViewBox>
</ScrollViewer>

private void UserControl_PreviewMouseWheel(object sender, MouseWheelEventArgs e)
{
     //Масштабирование
     int dx = e.Delta;
     int dy = (int)(dx * Field.Height / Field.Width);

     if (((ScrW.ScrollableWidth > 0 && dx <= 0)  || (ViewB.Width < Field.Width * 1.8 && dx > 0))
     || ((ScrW.ScrollableHeight > 0 && dy <= 0) || (ViewB.Height < Field.Height * 1.8 && dy > 0)))
     {
         ViewB.Width += dx;
         ViewB.Height += dy;
     }
     e.Handled = true;
}

_zoomTransform.ScaleX = _zoomTransform.ScaleY = newZoom;