Going back, your position is [-5, 0, 0]. Your friends position is [5, 0, 0].

The bulb starts at [0, 0, 0].

Your vector to the bulb (not normalized) is [5, 0, 0], because [0, 0, 0] - [-5, 0, 0] = [5, 0, 0].

Similarly, your friend's vector to the bulb is [-5, 0, 0] initially.

When you move the bulb north (z direction) by 10 feet, your vector is [5, 0, 10] and your friend's vector is [-5, 0, 10].

When the bulb is a mile away, your vector is [5, 0, 5280], and your friend's vector is [-5, 0, 5280].

If we normalize these vectors, they are very nearly the same. As the Z term dominates, the 5 versus -5 becomes a nitpicker's detail. For all reasonable purposes, you are looking in the same direction.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)