Jose writes:

It's not clear to me that becoming nearsighted is related to "strain" on
the eyes. My understanding is that it is due to the eyeball being the
wrong shape, and that presbyopia (needing reading glasses as we age) is
due to the lens hardening (and becoming unable to change its focal length).

Yes. Hyperopia and myopia are usually associated with asymmetry in
the shape of the eyeball. Presbyopia is presumed to be due to
hardening of the lens, although there is still some debate about this.

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