Where is the lower limit for a (baseball) strike?

Question

Yesterday, watching the Pirates-Mets game, I saw the Pirates' Andrew McCutchen called out on a third strike. The pitched ball was clearly over the plate, but "low." IMHO, it should, or at least could, have been called a ball.

In addition to being over the plate, the pitched ball must fall between the players' shoulders and knees. It was the "knees" part that confused me.

I was taught that "at the knees" meant at the level of kneeCAPS. Is that too high; that is, is there a definition of "knees" that is lower than the kneecaps?

Is it true that umpires have been informally expanding the strike zone, so that almost anything that is "over the plate" is called a strike as long as it is not in the dirt, even though it's technically "below the knees" (basically at the shin, as opposed to ankle, level)?

Could camera angles have caused a difference of opinion? That is, I saw a "flat" pitch "below the knees" through a TV camera, while an umpire, standing behind the plate, saw a "sinker," part of which was "barely" at knee level?

Answer 1

This page shows that the pitch caught the very bottom of the zone per PitchFx. The zone is technically from the letters to the knees. Granted, it's not always called that way, but this is how PitchFx is calibrated. This isn't an expanded zone, it's actually exactly the zone they are supposed to call.

Note that the zone is measured from the front of home plate, not where the catcher catches the ball. This pitch was very flat though, I don't have replay of it, but it doesn't look like it broke much from the pitchFx data.

As far as the true lower limit, Page 22 of the MLB rules indicates that the zone is below the kneecaps, not above them.