Look at it this way..
1. Optical Preview stops the lens down to your chosen aperture.
2. If you're using your kit lens (18-55) it has a maximum aperture of f3.5 at 18mm to f5.6 at 55mm (IIRC). If you assume every thing is linear then mid range on your zoom will have a maximum aperture of f4.5 at 36mm (it will be pretty close to that).
With optical preview you are expecting to see the viewfinder darken when you activate it, Correct?
3. If you set your zoom to 36mm and ask for f4, the camera is going to adjust for whatever the maximum aperture is. In the case of tic 2, that is f4.5. Wide open. When you activate the optical preview, the lens is already at it's chosen (adjusted) aperture and will not stop down any further.
If you want to see the effect across the entire zoom range, set your camera to something smaller than f5.6, like f8. Then at each zoom point, your camera viewfinder will darken when you activate the Optical Preview.