//zoom code
public Matrix GetTransformation()
{
    return
       Matrix.CreateTranslation(new Vector3(-_pos.X, -_pos.Y, 0)) *
       Matrix.CreateRotationZ(Rotation) *
       Matrix.CreateScale(new Vector3(Zoom, Zoom, 1)) *
       Matrix.CreateTranslation(new Vector3(_device.Viewport.Width * 0.5f, _device.Viewport.Height * 0.5f, 0));
}

//draw
_spriteBatch.Begin(SpriteSortMode.Immediate,
  BlendState.AlphaBlend, null, null, null, null,
  _camera.GetTransformation());

//for each tile
theSpriteBatch.Draw(mSpriteTexture, Position, Source,
  Color.Lerp(Color.White, Color.Transparent, mAlphaValue),
  mRotation, new Vector2(mSource.Width / 2, mSource.Height / 2),
  Scale, SpriteEffects.None, mDepth);