Perhaps I should have said that it is harder to see the size of the spot early from the hand the greater the height difference. The ball gets on you fast, so any difference is important. And it is harder for the batter looking at it from the same side (righty-righty or lefty-lefty) to catch it early.
In any case, I learned baseball in the 50's from a relative, Lester Stevenson, who'd been a pitcher in the minors in the 1910's. He was still coaching kids from time to time by the time I came around, spending summers in Wisconsin with my grandparents. I wanted to be a catcher, so I caught while he taught older kids how to pitch, with he and my grandfather giving me tips on my catching technique. Lester's specialty was the "nickel curve" (now called the slider) which he threw from exactly the same (lefty) release point as his fastball. He maintained that all pitchers move their hand away from their ears to throw a curve, so smart batters would pick it up. He considered his nickel curve to be the answer. His solution for this for kids throwing the curve, interestingly, was to turn in the windup, so the batter couldn't see your arm position 'till the release (Vargas's new "twist and shoot" motion, or, more famously, Warren Spahn's motion). We attended a lot of Milwaukee Brave's games (my grandparents and the Stevensons had a season box near the 1st base end of the Brave's dugout) with Lester giving an almost non-stop analysis of technique and strategy. When he died in 1959, I felt like I lost a friend, even though I was just a little kid.
I never became a real good ballplayer, partly cause I was small and had poor eyesight that required glasses, but I enjoyed playing and coaching through school and a Navy career and a second career as a school district technology geek. But I still remember summers long ago listening to why "Burdette had to be loading that ball" and why Warren Spahn was the greatest lefty of his (all) time.
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Sandy brings up a case-in-point that is news to me:
My fav eye surgery success story would be David Dellucci who had the surgery in November of 2004. His career line up to that point was: .261/.339/.418 (.757)
Then at age 31 and 32, he posted these lines:
.251/.367/.513 (.879)
.292/.369/.530 (.899)
He then reverted back to career norm and retired shortly thereafter. Was is causal? Impossible to tell. Too many variables in play. But it's intriguing. How much is better vision and how much is increased confidence?
A compelling example, especially in the SLG lines. Hits can fall in, but the 2005-06 Dellucci was obviously swinging with bad intentions. Your SLG doesn't go from below average to over .500 by accident.
Me, I'd go ahead and say it was close to 100% better vision and 0% increased confidence. ... he reverted, true, but at age 33, when reflexes are noticeably down for most hitters. In his age-33 season, the "dropoff" season he played about half the time in April, with a solid OBP of .340, but then was kicked to the bench, never played full time again and resumed the role of an over-the-hill baseball player.
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I'm sure that there are dozens or hundreds of players who had their vision corrected, and then hit much better -- and dozens or hundreds who didn't. It will be tough to prove anything in a court of law, but Dellucci is Exhibit A. Thanks San-Man.
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=== Bat571 ===
... didn't give his specific baseball background, but Dr. D can vouch (by proxy) for the general veracity of his comments below - you hear pretty much the same from any player you talk to:
Since the spin axis of a slider is coming towards the batter, you can see the "pole" as a reddish/pinkish spot (depending on the grip). It can vary in size - a tight one is often referred to as a "dime", while a looser one is a "nickel". A dime will break later and harder. Depending on the exact grip and snap when thrown, the spot is usually on the upper face as seen from the batter's box. Because of this, a very tall pitcher can throw a slider that is recognized later by a short batter because of the angle of flight relative to the plate. (I'm 5'7'' and couldn't see a slider spin initially from a pitcher taller than about 6'2" throwing from the old 14" mound).
I believe this is why some shorter batters have such trouble seeing the slider (think Jose Lopez). A curve's spin is more perpendicular to its travel and so is seen as a more general pinkish blur. The rate of spin can be judged, and its angle, but if a fastball is expected, adjusting is very difficult. However, a shorter batter with good eyesight can often see that a curve isn't spinning fast enough to break hard and punch it (again think Jose Lopez). Adair's "the Physics of Baseball" and several websites have better graphical explanations, but that's the basics.
I'd never in my life heard that a short batter could have trouble seeing the red spot in the spin, because too far on top of the ball from his angle, but I guess that makes sense.
Still, at Bat571 notes, hitters will tell you that they look at all kinds of things - spin rate is a big one, or maybe the release point is just slightly different - check the F/X charts on release points and you'll see that most pitchers let the ball go from an inch or two sideways on an offspeed pitch. Or there's some "tell" in the rhythm, or whatever. The red spot on the ball -- when they can get it -- is merely the "favorite," most effective, "giveaway" in most batters' opinions.
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=== They Call It EYE Ratio ===
A hitter needs intuition and experience to process a pitch in 0.20 seconds, and then hit it in the other 0.20. He seems to hit "critical mass" in his pattern recognition, right about the time he gets to old to launch the bat quickly...
James has pointed out that steroids are the "anti-aging" drug. Barry Bonds would have been a Hall of Famer -- about Ken Griffey Jr's level, roughly, adjusting for position -- without steroids. But starting at age 35-36, we got to see what would happen if you matched an old player's eyesight and pattern recognition, with his 27-year-old body.
My favorite Bonds year was not the 73-homer year, but the one after it. He had 198 walks -- 138 of them unintentional -- against only 47 strikeouts. It wasn't Bonds' power that I enjoyed watching. It was the fact that he seemed to see every pitch in slow motion.
I wonder if you gave Edgar, or Junior, or whoever, back their 27-year-old reflexes, combined with the way they saw the ball at 37, if you wouldn't get a bunch of those 200:47 ratios.
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