problem with blender switching from ortho to prespective during mesh editing

I ammodeling a mesh that requires lots of switching between front andsides, every times I switch and click on the middle mouse button forzooming or turning, being in orthographic, blender switch toperspective? That is very problematic because I cannot zoom or turnas I used to be in orthographic mode. I looked at the userproperties but other than the NDOF device setup in free and trackballthere is nothing else I could change… to stop that behavior.
Somebodyhas an idea?

Auto Perspective option

Cool thanks because it was really a problem for modeling… I wonder why it is the default? But anyway it works now. Thanks you for that.