xna - Orthographic Camera Zooming

In XNA I am using a spritebatch to render a board. I have created a camera class which can provide me a view and projection matrix. Currently I am supplying the view matrix to my sprite batch when I call begin.

I can get the view matrix to correctly move everything around and do some zooming in and out. My problem is that I want to zoom in on a specific point (for now the center of the screen) but currently zooming just zooms in on the coordinate 0,0.

Here is how I am constructing the view matrix;

        _ViewMatrix =
            Matrix.CreateTranslation(

                new Vector3(
                    (-_Position.X + _Width * _Zoom / 2),
                    (-_Position.Y + _Height * _Zoom / 2),
                    0) ) *
            Matrix.CreateScale(new Vector3(_Zoom, _Zoom, 1));

How could this be altered to make it scale correctly relative to the given point. Later I will probably want to zoom to the mouse position, but if someone has a simple example I'm sure I can work out how to adapt it.

Answer

The following code creates the desired result.

        _ViewMatrix =

            Matrix.CreateTranslation(-Position.X, -Position.Y, 0) *
            Matrix.CreateScale(_Zoom, _Zoom, 1.0f) *
            Matrix.CreateTranslation(_Width / 2, _Height / 2, 0.0f); 

This will cause the camera to zoom into the center of the screen instead of the top left corner.

My understanding is that this code moves the camera such that the object I want to focus on, which is at (Position.X, Position.Y), is in the top left corner (0,0), then I'm scaling by the zoom factor and then finally moving the camera by half width/height so the top left corner is now in the center.

The code in the question would move the object to the center of the camera, but scaling is always in relation to the origin which would therefore move the point further away from 0,0 and thus it isn't at the center any more.