Subtle reduction in visual function may be seen as reduction in contrast or color seen by the eye. Early or mild dysfunctions of the central retina or optic nerve can lead to such symptoms in a perceptive patient. These changes usually need to elicited by testing at an eye clinic. A sudden disparity in contrast or color between the two eyes, even if mild, usually indicates pathology to be investigated by an ophthalmologist.

Color vision may be genetically defective in some people due to one of the various forms of color blindness, such defects would be seen in both eyes and would not occur suddenly.

Other eye conditions like latent squinting or inflammation inside the eye can also lead to eyestrain. Persistent eyestrain needs to be evaluated by an ophthalmologist.