What, precisely, is "fog of war"?

[Rainmaker]

I have a serious problem after updating to 4.10.
I have trouble spotting contacts in the far-far distance as i was spotting them back in 4.09. I am wondering if 4.10 Fog of War option causes this?
My visibility is really poor now. I cannot spot airplane thats in 2.5 - 3.0 km from my position. I tried even to set my ressolution on 640x480 to see that giant dots, but still, without success. Any ideas???

btw. 4.10 needs alot of more testing before it goes "out". Alots of people complaining about stuff thats going on on 4.10. You will see a 10 year old kid flying on keyboard with spitfire and shooting down expirienced ace pilot of bf-109. I hope they will fix it before it ruins the game.