Being unable to focus can be a symptom of eye fatigue or strain, which is commonly associated with reading or being in front of a computer for extended periods, states WebMD. Some other symptoms of eye strain can be eye irritation and light sensitivity.
Another cause for an eye's inability to focus is blurred vision. Blurred vision can be related to certain eye issues, including presbyopia and hyperopia, as noted by All About Vision.
Hyperopia, which is also called farsightedness, is an eye condition in which people can see objects that are at a distance, but they cannot focus on objects that are close. Presbyopia can develop with age and is the inability to focus properly on nearby objects.
A macular hole in the retina is another ocular condition in which the affected eye sees distortions in objects, as stated by the National Eye Institute.