The knee-jerk answer to this question is always “age.” And while that’s not specifically wrong, it isn’t completely right. There are some grade schoolers that can’t see well up close. Not many, I’ll grant you, but the fact that there are any kids with poor near vision shows us that age isn’t the whole picture. So what causes it? (Full disclosure: the following discussion of hyperopia may not be what you’re looking for if you just want to get out of reading glasses. I understand if you’d rather jump to this article on that subject.)
What causes bad near vision?
So, for those of us still left — all two of us — what causes bad near vision? There was an article about what causes bad distance vision and, as you may have guessed, bad near vision is exactly the opposite. For folks who can’t see far away (that’s myopia), the image coming into the eye is maximally focused in front of the retina. Inside the eye, usually just a millimeter or two in front of the retina. The opposite of that, of course, with blur at near (that’s hyperopia), is an image that is focused just behind the retina. Outside the eye, usually a millimeter or two behind the retina.
When people can’t see well far away, they always have the recourse of bringing an object closer. The image in the eye will get pushed farther into the eye and eventually it comes into focus when the image focused on the retina instead of in front of it. You can imagine why that’s problematic if you’ve got hyperopia and the image is already behind the retina. Its maximum focus point is behind the back wall of the eye entirely. If you bring things closer, it only gets worse! Now the image has been pushed even further away behind the retina. This, incidentally, is the reason that people will start to hold things far away to read them when near vision starts to go.
Does it just come down to old age?
This is why age is a big part of the problem, but isn’t the entire answer. Aging of the eye only affects one player in this scenario, and it’s the one we’ve left out of the discussion until now. The natural, crystalline lens inside the eye. The reason most young hyperopes can see fine up close is because the lens is so flexible and good at autofocus. It can only work in one direction, but since that direction is pulling an image farther forward towards the front of the eye, hyperopes are in luck. The image was focused behind the retina, but the lens autofocuses and pulls it forward to the retina. Automatically! That’s pretty neat. It’s not neat for myopes. Their image was already in front of the retina, so autofocus can do exactly nothing to help with that.
This leads us to our last relevant term concerning near vision. That term is presbyopia. “Presby-” is actually the etymology for “old” because people weren’t politically correct or very nice when these terms where decided. Presbyopia refers to the stiffening of the autofocusing lens over time. As the lens gets stiff and stuck, those images at near that were once effortlessly pulled forward onto the retina are doomed to remain focused behind the retina. The autofocus has hardened and will focus no more.
Near vision becomes bad because your eye’s lens can’t move the focusing point forward onto the retina anymore. The more hyperopic you started, the earlier the lens will fail to complete its mission for you. It’s a thankless job, being a crystalline lens in the eye. It works really well for a long time, and the only way to tell is that you never noticed it. But when near vision gets bad, it’s time to assist it with readers, get it fixed with refractive surgery, or pass the menu to your nearsighted friend, who finally gets his day in the sun.