Every wonder why so many pros finish matches with challenges remaining? Me too. Why not challenge more calls, just for the heck of it? Especially given the results of UC Davis study (reported on here in the New York Times), which show that a vast majority of close-to-the-line shots that are called out were actually in:
The researchers identified 83 missed calls during the 2007 Wimbledon tournament. (Some were challenged by players and overruled, and others were later identified as unquestionably wrong through frame-by-frame video.) Seventy of those 83 calls, or 84 percent, were on balls ruled out — essentially, shots that line judges believed had traveled farther than they actually did.
It boils down to the way the human brain perceives motion. Since tennis balls fly faster than our ability to track them, our brains have to 'predict' where the ball will (or did) bounce; in doing so they usually overestimate the distance. Interestingly, line judges are actually trained to try to compensate for this:
Published instructions for United States Tennis Association line judges tell them to “focus your eyes on the portion of the line where the ball will land,” rather than attempt to track the ball in flight. “Get to the spot well before the ball arrives,” they are advised.
Of course, John McEnroe knew all this already, way back in 1981: