after aiming down the sight using a scope with the acog and red dot on top, and switching from red dot to the acog using the numpad "/" key, your view will be zoomed in, as if holding the 2nd mouse button down.
to revert back to the none zoomed in mode (when not aiming down the sight) you have to press the numpad "/" again. this then means when you go to aim down the sight again it will be set back to the red dot sight.

essentially, the optics mode is effecting the zoom, when not using the scope