Good question,
I'll take a stab.
Even the very best glass (Swaro HD, Leica APO etc.) has a sweet spot (practical upper level of magnification per the conditions.)
Heat mirage, weather, etc. play such a factor that most high power scopes don't see that upper echelon of their magnification unless one is watching hummingbirds 50 yards from the kitchen.
For me that average sweet spot was about 35x magnification with my 16-48x Leica 62mm televids for sheep hunting. Anything more than that just created too much distortion.
Less is often more when if comes to magnification. But it is nice to be able to 'crank it up' (40-60 power) when very close to a target. That is where it is usually most practical for a great distortion-free view.
My two pennies, and I've had some nice glass.
