He, conveniently, added a little diagonality to her camera angle, apparently realizing that she had to shoot diagonally if she shot the Moorman photo (which she didn't). But, it's ridiculous. It might be plausible if the limo were going the other way, as I have indicated with my arrow. But, Mary was waiting all morning to shoot the Kennedys, and she was ready to go from the moment they made the turn at the top of Elm. The idea that she would let them go past her- past perpendicular- is preposterous. So, it's one thing to say that she may have shot a little bit diagonal going the other way, opposite to this. That is not only possible, but it's probable. Because: who would wait until the last split-second to capture the picture? And let's remember that in the Zapruder film, we see her physically turned the other way with the camera in place.
So, there she was, poised and ready to shoot; her finger on the shutter button. But, you say she decided to forego that opportunity and wait all the way until they had passed her, and she was now shooting them from behind? That she tracked them through all of that, and just waited, waited, waited? Well, my response to that is just two words, and the second one is YOU.