Perpective View

As avc81 said, the orthographic projection is what you want. What you do not want is the perspective distortion, yet this is exactly what the perspective projection does. Either you use the orthographic projection or you implement an algorithm known from billboarding such that the cube always faces the camera. The first one is the way to go.

i implement zoom by means of the Matrix4d.CreateOrthographicOffCenter function

every time there is a "zoom action" i recalculate left,right, top, bottom values and then i call that method.

Basically a "n0-zoom" scene is given by

 Dim right As Double = _scenewidth * 0.5 
 Dim top As Double =_sceneheight * 0.5 
 Dim left As Double = - _scenewidth * 0.5 
 Dim bottom As Double = - _sceneheight * 0.5

while a "zoomfactor-scene" is given by

 Dim right As Double = _scenewidth* 0.5 * _zoomfactor
 Dim top As Double = _sceneheight * 0.5 * _zoomfactor 
 Dim left As Double = -_scenewidth * 0.5 * _zoomfactor
 Dim bottom As Double =- _sceneheight * 0.5 * _zoomfactor

where zoomfactor must be >0 (just play with this value and see what happens ;))

Hope this helps

Thx, i got it now.
My second problem now is to fix the object at the mouse while i move it (right, left, top, down). Because the ratio changes in every zoom-position. I move the object by Transformation. I calculated the moveDivFactor based on the screenSize:

like this:
800x1 + 600x2 = 36
800x1 + 800x2= 48
...so i got x1=0 and x2=0,06

however it worked fine but only when i dont zoom

transformation.X -= e.XDelta / (float)(this.Height * _moveDivFactor );
transformation.Y -= e.YDelta/ (float)(this.Height * _moveDivFactor ) ;

But know when i zoom the proportions change and the object isn´t fixed at the object 1:1 when i move it. Do you have any clue?