Hmm, not sure why you get varying readings at different zoom levels.
I know my attack scope works just fine at 1.5x-6x-15x, I rarely if ever use the observation scope unless the attack scope is damaged but I bet it's OK too.
Are you using any UI modifications? If not, try one and see if that fixes it.
I suggest something like MaGUI. It has its quirks, like auto-closing tubes when you select one from a certain view (UZO I think).. But the optics are accurate and if I stay away from the UZO tube selector switches, mine seem to stay open. Also, you can actually pan around with the scope while the game is paused. I like that, but many would call it cheating
Actually I believe the tube thing is a stock SH3 bug, and I use the TDC selector switches to get around it. Some mods fix it, some don't but I always do switching on the TDC out of habit.