This is not in my usual style. This is a little visualization tool I wanted to have. It's likely this will be improved upon over time. I just wanted to release it to be able to show it to friends and family.

Other than "glasses work by bending light and usually make us see clearer", the way they interact with our vision is not common knowledge. The goal here is to help you understand the effects of glasses on your vision and maybe help you understand how prescriptions impact you.

I'm not an optometrist, but I learned a lot on the subject. There are of course simplifications in here.

Your eye is a lens that tries to focus (to bend) all of the light coming from a single point in front of it to a single point on its retina (it's back surface).

What makes this hard is that that the closer an object, the stronger the lens must be to focus it at the same spot. Yet we can see clearly from many distances, not just from a specific once. The eye accomplishes this by having a lens than can change strength: the crystalline lens.

The strength of lenses is measured using diopters. Without going into details, what interests us is the size in meters of a diopter... It varies based on the distance!

Move your mouse in this diagram. The top line is a distance in meter from the left. The second line is consecutive 0.25 diopters. When you move the mouse, you will see the highlighted the distance when going closer (left, aqua) or farther (right, orange) by 0.25 diopter.

Notice how the aqua side is smaller than the orange one? When you get close to half a meter, the sizes gets tiny! And at the other extreme, they get huge! In fact, at 4m, adding 0.25 diopter takes you do infinity!

The crystalline lens of the eye is great, but is has limits. This means that the eye has both a minimum and a maximum distance it can see. Try it, bring a piece of text close to your eye. At one point, it will get blurry.

How much the eye can change strength lower as you age. Wikipedia says "The age-related decline in accommodation [the strength change] occurs almost universally to less than 2 dioptres by the time a person reaches 45 to 50 years"

So by the time you reach 45 to 50, your eye is likely to be able to adjust by less than 2 diopters.

Lets visualize this. Here, the mouse location represent your minimum distance and the orange zone represent 2 diopter farther: your optimistic but likely maximum distance.

If your mouse is close to the left edge, such as minimum distance of 0.25m / 25cm, then the maximum distance would still be quite close at 0.5m / 50cm.

If you move your mouse to 0.5m, then finally, you get to see clearly at any distance... As long as that distance is above 0.5m.

Vision glasses

Ok, so how do glasses fit into this? Well they are lenses and are rated in diopters! And when you have multiple lenses, the effect is similar to a single lens with the diopters added. This means when you wear glasses, your whole vision range, minimum and maximum, get shifted by that many diopters.

The two main categories of people wearing vision glasses into are:

• nearsighted (myopia): Without glasses, they see clearly objects that are close, but those that are far are blurry.
• farsighted (hyperopia): Without glasses, they see clearly objects that are far, but those that are close get blurry.
  As mentioned, an object that is really close to the eye will be blurry for anyone. The idea here is about comfort, if you can't see clearly an object that is half a meter (18 inches) away from you, this can be quite annoying.

For nearsighted people, glasses will normally have the strength required to bring their vision's maximum distance to close to infinity (from 10 meters, infinity is only 0.1 diopter away). For younger people, their vision's minimum distance will remain low enough to not even notice that it moved.

For farsighted people, it's all about that minimum distance. It's too far! But unlike nearsighted people, there isn't a clear limit to how much it can be corrected. With strong enough glasses, your minimum could become 10cm. Instead, the goal is to move the vision close enough for most close work to be comfortable. This may vary based on what you do.

Progressive lenses / multi-focal

TODO: I lack expertise to continue here...