You may have seen people using colored films or tinted glasses when they read. For a small group of people these are critical aids to their ability to read.
Complains of text "moving around" on the page
Complains of other distortions of the text
Rubbing of the eyes when reading
Feeling of nausea looking at black text on a white background
Quick fatigue when trying to read
General discomfort with bright lights and strong contrasts
Marked improvement when colored overlays are placed over the page
We think of our eyes as being like a digital camera, with each "rod and cone" being like a pixel. The reality is quite different. There are around 100 million rods and cones on each retina and only 1 million nerves passing out the back of the eye. So a lot of processing is happening right in the eye. Indeed, your eyes are actually a special part of your brain that has been relocated outside of your skull.
What the eyes mainly detect are changes of intensity. They are excellent at picking up pattern and your brain then takes that information to present to you the impression of a three dimensional space around you. That perception is entirely fabricated in your visual cortex from very different information coming from your eyes, rather like your own mini version of the film The Matrix.
Humans are particularly good at this ability to spot a pattern. Indeed, we can even see patterns when they are not there. Have a look at Kanizsa's Triangle:
Most animals can only see movement clearly. That is why a good technique when faced by a large carnivore is to stand absolutely still. And in a bullfight the matador is pretty safe if he can stay still. The bull will always follow the movement of the cape (not the color) and loses interest once everything is stationary. Rabbits had the right idea too, until they started trying to cross roads! In contrast to that, we can spot a stationary tiger's face in the long grass, hopefully.
Like almost all things, there is a range of ability with this, which follows a "normal" distribution. At the extreme end of that distribution people start to find strong contrasts cause discomfort. That affects their ability to focus comfortably on black text with a white background. It shimmers, a bit like green text on a red background for most of us:
The solution to this one is simple and obvious, once you understand the problem; you just reduce the contrast. That can be done with a coloured overlay, printing on coloured paper or using tinted glasses. You can also adjust most monitors to reduce their contrast and give the white a tint. This page has a tint to it too, to reduce this issue and you can change the tint in the top right corner of the screen.
I have seen some people use such a deep color that I can hardly read the text. It can look like this:
Some optometrists specialise in testing for this issue and can provide colored overlays and tinted glasses. Have a look in our listings here on the site. You can search for colored overlays or "Irlen Syndrome" in your search engine.