If you spend a lot of time using a computer, reading, or watching TV you could be straining your eyes. This happens because for the majority of the day your eyes are focused on objects that are close to you, so your eyes become conditioned to seeing close objects. As a result of this the eyes becomes less able to see distant objects clearly. When the eyes are relaxed the natural focus point is about 7 feet. From this you can see how much tension is put on the eyes to focus up close.
One rule to apply to minimize the risk of eye strain is to use the 10-10-10 rule. When you are working on any type of activity where the eye is focussed closer than 7 feet you should apply this 10-10-10 rule. For each 10 minute of close activity you should focus on an object at least 10 feet away for at least 10 seconds.
The more you do this the less likely you will experience any deterioration of vision.