It appears you are:
- Not blinking enough (you should be blinking 16-20 times a minute apparently), dry eyes are a major source of eyestrain (especially in women)
- Staring at a flickering CRT monitor or a dimmed LCD screen. LEDs used to backlight LCDs, can’t be dimmed. So LCD manufacturers fool the eye into thinking LEDs have been dimmed by flickering them (eye interprets flicker as darkness).
- Not staring at things far off in the distance periodically. Genetically our eyes are built to alternate between focusing on objects close to us and in the distance. If you have a white collar job you are probably using your day to stare at a computer screen close to you, rather than that gazelle in the distance.
Apparently in theory you shouldn’t be getting eyestrain from LCDs or iPhones at all. I found this strange because I have both and I definitely get eyestrain from them occasionally.