It's the pwm dimming that causes eyestrain. Not everyone, but a sizeable portion of the population.
I found out about it from my doctor back in 2011 when my 11" MacBook Air was causing headaches. The screen blinks on and off at 240Hz which is enough for your eyes to recognize and try and adjust; but it's not enough for your brain to register what you are seeing.
I used a W530 later on and it was very bad, headaches at anything other than 100% brightness.
Lenovo fixed the issue in 2016 on most of their laptops, and the Retina MacBooks have never had pwm dimming.
Notebookcheck.net tests all their reviews for pwm dimming.