Often the camera isn't lined up correctly to tell if the ball has crossed the goal line (It's a 2D image of 3D space, so you have to take one of the dimensions out).
Imagine a camera at, like, the 2-yard line, with the goal line to the left of center. The white goal line would be slanted on the screen, like: /
The closest edge of the goal line is in the bottom left corner, the farthest edge would be closer to the center of the top of the screen. With the ball somewhere in mid-air trying to cross, you can't really tell if it does. Like, in
this picture, you'll never be able to tell from this angle if the ball crosses the goal line.
The correct way to do it would be to mount, permanently, a camera exactly over the line, centered, and facing directly down the line. Probably 10-15 feet away from the pylon and 10 feet in the air, preferably on both sides. The closest edge of the goal line would be bottom dead center, and the farthest edge top dead center. Like: |
That way, every pixel on one half of the screen is the end zone, and every pixel on the other half of the screen isn't. It's a zero-parallax view, or single-point focus. The goal line is exactly vertical, so you know easily if the ball crosses it at some point.
It is done correctly in soccer (see how the absolute vertical line makes it easy to tell the ball has crossed the goal line, whereas if it was at an angle, it's harder), horse racing, auto racing, cycling, track, cross-country skiing, speed skating, and even dog racing... pretty much every other sport that has a fixed "finish line" or "goal line".
In addition to a pair on each goal line, another pair should be fixed along each sideline, at a zero-angle so we can see exactly when someone (or the ball) goes out of bounds. Look at
that Jets non-TD from Austin Sefarian-Jenkins, due to the odd 45-degree angle you can't tell in space when the ball crosses out-of-bounds, and if he has control then or not. Ideally, you'd be able to view that play from a camera fixed from somewhere above & behind the back corner of the end zone looking dead-on to the 50 yard line, with the sideline a vertical stripe in the center of the screen: | and add in a second angle at exactly 90-degrees centered exactly at the goal line, and by watching those two fixed cameras you'd see everything you needed to give the correct call of the play. (I'm told that the NFL headquarters has the capability, during instant replay, of "synching" two or more angles together so that if, say, you can tell exactly when a player's knee hits the ground in one angle, that you can see exactly that moment of time in another camera and see where the ball is, if it's not easily visible in the first. (Another piece of trivia: TV cameras are synched up precisely, in order to cut from one to the other without a glitch frame, and, technically, are within one 125-millionth of a second of each other))
The thing is these fixed cameras can never "pan" left or right, and shouldn't really "tilt" up or down either, they should be mounted and still, like security cameras.