i have recovered from 8 dioptries of shortsight by tensing the zone between the eyes

[ATAG_Dutch]

I think you have some terminology confused here raaaaaaaaid, me old mate.

The muscles of your eyes which control convergence, similar to the convergence of the guns on your aeroplane, are linked via the autonomous nervous system to the ciliary muscle which controls the 'accommodation' properties of the lens of the eye.

Consequently, when you look with both eyes at the end of your nose, your eyes turn in, but at the same time, your ciliary muscle fattens the lens of your eye thus decreasing it's 'focal length' in order to attempt to be able to focus the image.

Then once you're looking at it, your 'dominant eye' takes over, and the brain suppresses the image from the other eye, giving you the illusion that you can see distant objects in focus.

Whilst you're doing this, you'll look pretty foolish for one thing, but for another thing it makes you 'longsighted' not 'shortsighted' (Or Hypermetropic rather than myopic, if you like. ).

If you were genuinely 'shortsighted', or myopic, no amount of muscle work would help. Although a pinhole in a piece of paper would.