A STRIKE is a legal pitch when so called by the umpire, which—
(b) Is not struck at, if any part of the ball passes through any part of the strike zone;
The STRIKE ZONE is that area over home plate the upper limit of which is a horizontal line at the midpoint between the top of the shoulders and the top of the uniform pants, and the lower level is a line at the hollow beneath the kneecap. The Strike Zone shall be determined from the batter’s stance as the batter is prepared to swing at a pitched ball.
The OBR also provides this picture.
However, the picture is misleading. From the literal meaning of the rule the strike zone is (1) the area over home plate; (2) which passes any point from the midpoint of the top of the shoulders and the top of the uniform pants, and the hollow beneath the kneecap. That is a three-dimensional area that should been seen more like this:
According to the black letter reading of the rules, the roundhouse curve that passes through the strike zone at the midpoint line at the very back tip of the plate is a strike. Sometimes the catcher may stand up on that pitch.
According to the rule, the nasty splitter that passes through the strike zone at the hollow beneath the kneecap and over the front edge of the plate is a strike. That pitch likely bounces in the dirt.
While not a professional umpire myself, I have heard from major league umpires that they do not consider the above examples to be pitches that “pass through the strike zone” even though the rule specifies that it is a strike if any part of the ball passes through any part of the strike zone. The reasons for this depend on who you ask and all are based on perception and custom.
The New York Times has recently reported that “[a]n improved camera system to monitor umpires’ calls of balls and strikes will be used in all 30 major league stadiums starting opening day.”
“The new system, called Zone Evaluation, relies on pitch-tracking data already collected by cameras in all 30 parks and distributed through applications on MLB.com and iTunes. Zone Evaluation software will rate umpire performance more quickly and accurately than QuesTec, according to Mike Port, baseball’s vice president for umpiring.”
So will the strike zone change because of technology? There is no doubt. The real question is whether it will change for the better. And what about that high round house curve or the low splitter? Will these pitches become more widely used because umpires will be encouraged to call these “strikes?” How does this improve the game? It sure makes the calling of balls and strikes more controversial. I can't imagine this being a good thing.