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Line Calls — Who Can You Trust? Joe Dinoffer Google “bad line calls and tennis” and John McEnroe is in the second listing. When McEnroe was playing the pro tour, it was electrifying, and not just because of his tennis talent. He regularly created the kind of drama that also made the “Jerry Springer Show” so popular. People like controversy, and McEnroe created enough controversy to satisfy the appetite of a street fighter.
I don’t know why people are entertained by over-the-top behavior, but they are. Clearly, McEnroe trusted his own vision on close line calls way more than he trusted anyone else’s. And he expressed his opinion with tirades and tantrums. You have to wonder if he missed the chapter in high school science that dealt with the human eye and what it can and cannot see. Part of this article will include what Mac clearly didn’t learn in high school. So, what percentage of the time was McEnroe correct when he challenged line calls?
To reality check your own visual capacity, have a friend place a dozen balls around the baseline, randomly placing some just inside the baseline, some touching just a part of the baseline, and some just barely outside the baseline.Next, stand on the opposing baseline and try to judge which balls are in or out and have your friend keep score. You may be surprised. Even when you are standing still, and even when the balls are stationary, it will be very difficult to precisely and confidently see whether each ball is in or out. Why? Two Reasons We Can't Make Accurate Line Calls
There are two scientific reasons we cannot tell whether a ball hit near the opposing baseline is in or out.
Reason #1 — Distance Let’s start the discussion by looking at a standard eye test. We’ve all seen the Snellen Eye Chart, the original version of which has been in use since 1862! If you can read line 8 from 20 feet away, you have 20-20 vision. So, if someone with 20-20 vision looks from baseline to baseline (78 feet), how accurately can he or she judge whether a ball that lands close to the line actually hits the line or not? To help answer this question, the Snellen Eye Chart says a person with 20-20 vision can read the line of letters with a font height of .56 centimeters (less than 1/4th of an inch). At the 78-foot baseline to baseline distance, the corresponding height of the 20-20 vision line of letters becomes 7/8ths of an inch. While a tennis ball is about three times that size, it is the space between the ball and the line or the overlap of the ball and the line that gets too small to see.
Reason #2 — Ball Speed A ball traveling at 50 mph will stay in contact with the court 2-4 milliseconds, that is, if we use the commonly accepted dwell time for a ball in contact with the racquet strings. Note that 4 milliseconds is only 1/250th of a second. Since the human eye sees movement cycles only as fast as 1/60th of a second, it is clear that we cannot actually “see” a ball hitting the ground. It happens too fast. Our brain connects various dots to create a line of flight, but we really don’t see the details. Here’s a slow motion clip shot at 600 frames per second that shows how short an event the bounce of a tennis ball really is. Even Spiderman Would Have Trouble Calling Lines! Based alone on the two reasons above, is it possible to clearly judge whether a ball you hit landed on part of the opposing baseline or just missed? Even John McEnroe would have to admit the impossibility of seeing this event! Heck, even a comic strip hero like Spiderman would be challenged to make accurate line calls in tennis. To further the argument that accurate line calling on close calls is a very difficult proposition, we have to also consider the many variables that compound the difficulty. These include:
The short video clip, courtesy of the ITF, shows a close up of a ball bouncing and skidding on a flat surface. Imagine trying to confidently know if your shot touches a little part of the line or just barely misses it when you are standing on the opposing baseline! . Hawk-Eye Technology While discussing line calls, we would be remiss in not discussing Hawk-Eye technology and its margin for error, which is +/- 3-5mm. People think Hawk-Eye technology is mistake-proof, but this is simply not the case. Just one of the reasons why Hawk-Eye makes mistakes is that tennis balls compress every time they bounce. And, that compression varies from shot to shot and from surface to surface, depending on ball speed, spin, and trajectory, among other factors.
The ITF video clip (above) showing the “skid” that balls make displays this challenge. So, while Hawk-Eye is clearly a big improvement over human visual capacity, it is still far from perfect. But, while it is true that Hawk-Eye technology may have a margin for error of +/- 3-5 mm (note: 5 millimeters = less than ¼ of an inch), it’s not quite as bad as it sounds. It is still a big step up in accuracy from John McEnroe’s often irate line call opinions when close calls didn’t go his way. Conclusion So, in that fateful science class which McEnroe probably missed, he might have learned that it is totally illogical to think he could call balls on the opposing baseline if they land close to or just barely on the line. He would simply be playing “connect the dots” in deciding whether a ball landing relatively close to the line is actually “in” or “out.” Sorry, John, these are just the facts. If you are completely disillusioned about getting a fair call on a tennis court, whether intentionally called wrong or not, you can take home at least one thing from this article. In recreational play, the rule of etiquette on line calls is “Give the benefit of the doubt to your opponent.” Translation: If you do not see a ball bounce clearly out, call it in. Chances are your opponent will treat you with the same respect and you will sleep better at night. And that just might help you play better the next day.. Your comments are welcome. Let us know what you think about Joe Dinoffer's article by emailing us here at TennisOne. |
