Don’t read in the dark.
— 1940's Mom
Don’t sit so close to the TV.
— 1980's Dad
Don’t hold the iPad so close when you watch Netflix.
— Today's Parent

Parents worldwide have imparted these pearls of wisdom on their children to keep them from "ruining their eyes"... or according to my mother, making square eyes.  The truth is though, that there is no evidence to suggest that any of these activities are actually all that bad for the eyes, with the exception that they all would make the eye have to work harder.

A typical eye that has perfect vision at distance (termed emmetropia), is an eye that in its most relaxed state is able to focus light rays originating from objects 20ft away and further.  This is an eye that wastes very little energy when viewing far away objects since the eye naturally is focused at that distance when totally relaxed.  One can imagine this might have been a good setup at certain times in early human history when food was scarce and available energy for the body was low. In these prehistoric times, the emmetropic eye could still scan the Savannah for predators while wasting very little energy in the process. 

Like any optical system though, the eyes can only have one focal point at a time.  So if the eye is naturally focused on the horizon (like in the example above), then closer objects are by definition blurry.  For the eye to see the close objects clear, it needs to squeeze a muscle in the eye (the ciliary body) and change the shape of the internal crystalline lens.  This process consumes energy, but as long as the energy is available and the crystalline lens remains flexible enough to change its shape, the eye will change focus seamlessly from long distances to up close pretty much on demand (you're likely doing it now as you read this article).  One can imagine in prehistoric times a person would spend most of their day wasting very little energy while viewing objects at long distances, with only brief periods of energy expenditure when changing (and maintaining) focus on closer targets (like starting a fire, sharpening tools, or eating food). Contrast this with present day viewing habits, where it is estimated that we spend approximately 80% of our day viewing objects that are within arm's length.  This implies that for only 20% of the day are the eyes in a relaxed state, with the rest of the time them being under muscle strain and expending energy.  This is why our eyes can feel tired, fatigued, or even blurry at the end of a day.  And the system only gets more inefficient as we get older, wasting more and more energy, eventually culminating with the need for reading glasses in our 40s (termed presbyopia).  Therefore our focusing system is at it's most efficient when we are young, which explains why children have the ability to sit very close to the TV or hold the iPad very close to their faces without too much difficulty.   Even though they are squeezing the ciliary body in their eye, and expending energy, the system is so efficient that they barely can perceive that they are straining.  And since they don't immediately feel this strain and we all know that holding objects closer to our eyes makes it appear bigger, brighter and more vivid, children are motivated to continue this viewing behavior.  What kid wouldn't want to watch Paw Patrol like this?.  

However, even with their efficient focusing systems, viewing close targets such as an iPad or a TV for extended periods will likely lead to some eyestrain, since the ciliary body will eventually get tired, which may manifest as headaches, fatigue, or irritability; but definitely not blindness nor, at least directly, lead to needing stronger glasses when they get older.  

I say not directly, because studies show that sitting inside the house and viewing a TV, an iPad, or even a book, no matter at what distance, can lead to an increased need for glasses in the future.   Not from the strain though, but because staying inside the house means the child is not outside playing, where the sun would be shinning and causing the brain to release dopamine, which seems to play an integral role in the eye developing properly.  So, I guess in theory sitting too close to the TV can be harmful, in the way that not getting the proper amount of sun exposure can be harmful to a developing eye.

Dr. Burke is an optometrist practicing at Calgary Vision Centre.  If getting square eyes from watching too much TV was possible, his would be square.  Opinions above do not constitute medical advice, and readers should consult with their optometrist if they have questions or concerns about their eye health