eyes - Cause of short sightedness/ far sightedness

The physical shape of the eye and lens are as @RoryM points out, the reason we become near/farsighted.

The most common cause of farsightedness is pretty well known. The lens of the eye allows us to focus as a ring of muscles pulls outward and flattens the lens, allowing it to focus on closer objects. As we get older the lens of the eye becomes stiffer and inflexible, which means that the muscles that pull on the lens can't flatten it as easily and focusing on nearby objects becomes difficult or impossible. This is why we need to get bifocals or reading glasses as we are older.

The reasons for nearsightedness are 'natural changes in the shape of the eye'.

Mentioned briefly in the WebMD link above is that close work (e.g. reading, web surfing, phone videos, reading and reading, etc) tends to cause nearsightedness.

25% of adults in the US and 40% of adults in Asia are myopic and it does look as if there is a strong genetic component to myopia. If neither parents have myopia, the incidence is only about 7%.

I'm probably going to get ding-ed for this, but I am going to offer alternative explanation, which makes good scientific sense to me, so hear me out.

This seems like a poor adaptive phenotype for humans who are otherwise so optimized for survival. Why would we naturally lose our ability so young (often before the fifth grade)? So many people who have myopia are so impaired that they can't find their way around their own house (or legally drive) without glasses/contacts/corrective surgery.

This is supported by studies which show that non industrial societies have almost no natural myopia compared to industrialized ones. This is just one example reference, not a survey of all evidence.

Physiologically the explanation for myopia is this: in the relaxed state, the eye is focused at the horizon (infinity). If one habitually focuses on a book or screen within 1-2 feet from your face, the muscles might well adapt to a set point that is closer than infinity. This would make myopia an adaptive response to modern life.

Eye exercises are proposed as a way to correct myopia. This is a bit controversial - some say they help, others say they do not. I would not be surprised that myopia developed in youth (8-15 y.o.) when the eye and body are developing can't be overcome by eye exercises when you are 25-30, but the jury is still out.

There are enough free descriptions of eye exercises out there that you don't really need to pay for them. Getting young students outside to look around once in a while doesn't sound like a bad idea either, given this.

If you are going to down vote or complain, I would appreciate it if you please cite counter reasoning other than 'this is not a popular explanation'.