Comment by dTal

Focus is distinct from convergence - convergence is how much you have to cross your eyes to look at something, but focus is where muscles squish and stretch your eyeballs to change the distance of your retinas from your pupils. Just like a camera, if your eye is not focused at the real distance to subject, it will look blurry because your pupil is not a perfect pinhole, but has area (so a single eye is already seeing the same object from slightly different angles, on either side of the pupil).

Usually your brain learns a strong correspondence between focus and convergence, but this can be unlearned quite easily, and indeed must be in order to view e.g. VR, 3D films, Magic Eye pictures, etc... - all of which encode 3D information through convergence, while requiring your eyes to focus on a fixed plane.