An explanation in pictures

If you fix a distant object (the Moon ...) and if you extend one arm before your eyes, the index up:

We must keep this faraway look, parallel, and look down on the screen images, which seem twofold:

When you comes to position your eyes, the eye spacing is automatically adjusted to reconstruct the stereoscopic image and give you a "natural look":

Note: Usually, you associate convergence and near focus (focus on nearby objects), but there is no difficulty to associate the latter with the divergence, the muscles of the orientation of the eyes are very distinct from those of the accommodative.