Eyesight is perfect when you see things dead on most clearly. This is called Center Fixation and means whatever you see in the center of your vision is what you see the best. Perfect eyesight works when you look at things straight on and don't have to move your head or eyes to the side to focus.
When your vision is off, you actually look at the side of an object to see it clearly. Test this theory and see where you currently stand. Find a piece of paper with small print on it. Look at the words straight on and then turn your head to the side until you can read the print. Move your head one way and then the other until you find the best position for reading the print perfectly. Are you straight on or to the side?
The retina is located in the center of the eye and is full of small light sensing rods. This area is called the fovea centralis and is the area of the eye created to have the best line of vision. If you are tilting your head to the side to see properly, the fovea centralis is not working correctly. The fovea centralis may actually even be turned off. As a defense against outside dangers or stress, this area can shut down to protect itself. If it is turned off, your vision will not be as perfect as it can be.
Vision can be affected by our feelings and emotion. Center fixation can be off because of negative feelings inside our minds. Negative memories, experiences, or people can force our eyes to turn away and avoid head on contact. A lot of vision problems are 'all in our heads'. Learning to deal with tough situations in a relaxed way can be difficult, but it's necessary for good vision. Every situation should be looked at and dealt with head on, not with a turned away from.
Simply learning how the eyes work can help us to use them properly. Training the eyes to focus on things straight on will help avoid strain and keep them relaxed.