Batrider's vision is reduced from 1800-800 to 1200-700 when he is flying in Dota 1. ( http://www.playdota.com/forums/showthread.php?t=1295515 )
1) Get batrider with firefly during daytime
2) Go into an open area where you can see your whole sight distance, with no allies providing vision
3) Observe fog boundary and use firefly
Vision just becomes completely unobstructed, no change in sight radius
During daytime the sight radius slowly reduces to 1200 flying vision, and to 700 at night.