Because of the way video cameras work, it's a lottery whether or not they have a picture of the moment the ball comes into contact with the foot. They may have an image just before or just after but not the exact moment. The frame rate is only half the story. The 'shutter' speed comes into it too. If the camera speed is very fast, they might have a few images of ball on foot. Generally, for the moving picture to appear normal, I understand the speed has to be at least double the frame rate. So at 50fps the speed has to be 1/100th second, at 120fps, 1/250th sec. 1/100 th sec will give a blurred still image of an object in motion. 120 frames at 1/250th sec gives you a recording of about half a second's worth of action. So you've got a 50/50 chance of recording the actual moment the ball comes into contact with the foot. Sometimes they'll have the right image, sometimes they won't. So it's clearly not suitable for offside unless we all accept that you win some, you lose some. In that case what's the point of it? It just absolves referees of all responsibility and gives the final word to somebody far away playing with some software application to advance, reverse, sharpen, focus, zoom and so on.