xna - How can I use a camera matrix with different resolutions?

I created a little jump'n'run game. The character(Mario) is always the center of the viewport. The game is running correctly if I use the resolution 800x480 pixel. But when I use another resolution(for example 1280x768), the sprites in the viewport look completely different to the sprites in the 800x480 viewport. I want that the viewport(and the sprites in the viewport) looks always the same, on every single resolution.

How can I do that? Should I change something on my camera matrix parameters or what should I change so that the viewport and the proportions of the sprites look always the same?

I use Monogame to run the game on different Windows Phone devices.

My camera class:

 public class Camera 
{ 
    public Vector2 Cameraposition; 

    public Camera(Vector2 cameraposition) 
    { 

        Cameraposition = cameraposition; 
    } 

    public void Update(GameTime gameTime, Vector2 CamPosition) 
    { 
        Cameraposition = CamPosition; 
    } 

    public Matrix GetMatrix() 
    { 

        return new Matrix(1, 0, 0, 0, 0, 1, 0, 0, Cameraposition.X, Cameraposition.Y, 1, 0, -Cameraposition.X, -Cameraposition.Y, 0, 1); 
    } 
}

In Game1:

//I use the camera in Game1 like this:
//At the beginning(in LoadContent): 
Vector2 cameraposition = new Vector2(PlayerStartpos.X - GraphicsDevice.Viewport.Width / 2, PlayerStartpos.Y - GraphicsDevice.Viewport.Height / 2); 
camera = new Camera(cameraposition); 


//Updating the camera position
 protected override void Update(GameTime gameTime) 
    { 
        player.Update(gameTime); 
        Newcameraposition = new Vector2(player.Playerposition.X - GraphicsDevice.Viewport.Width / 2, player.Playerposition.Y - GraphicsDevice.Viewport.Height / 2); 
        camera.Update(gameTime, Newcameraposition); 

        base.Update(gameTime); 
    } 


//Drawing: 
spriteBatch.Begin(SpriteSortMode.Deferred, BlendState.AlphaBlend, SamplerState.AnisotropicClamp, DepthStencilState.None, RasterizerState.CullNone, null, camera.GetMatrix()); 
 ...

Answer

Your question is not an exact duplicate, so I won't try to close it. But it's close enough that I feel comfortable in simply sending you over to my old answer on Supporting Multiple Resolutions.

That answer is specifically for the XNA Platformer Sample, so you should probably download a copy of that to play around with to get a feel for how the code in my answer works. Then you can apply it to your own game.

---

Note, in particular, how I use matrix multiplication to combine two matrix operations, as mentioned in comments on your question:

Matrix camera = Matrix.CreateScale(scale, scale, 1)
        * Matrix.CreateTranslation(translateX, translateY, 0);