Degenerate — the series
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Far enough away, a pair is one eye
Hold the baseline and walk the scene away, and the parallax a single homography cannot explain falls as the distance to the power −0.968 — one over the distance, which says the ratio of baseline to depth is the whole of it. The recovered translation direction follows it down, from 3.3° at four metres to 74.5° at two hundred and fifty-six.
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A longer lens buys parallax and not the direction
Walk a pair away from a scene and the parallax a homography cannot absorb falls. Lengthen the lens to keep the scene filling the frame, and one power of that loss comes back, exactly. The recovered direction between the cameras does not come back with it. At the same parallax in pixels, a long lens is three times further from the answer, because what fixes a direction is the angle the marks span and no lens can widen that.