It's been discussed many, many times. I have an iphone 4S. One of the best displays ever made I'm told. I get eyestrain. I've played with the settings and colours etc etc etc. I get eyestrain.
I have a Sony PRS-650. No eyestrain. Ever.
There's a difference to me despite what the study suggests.
Many a man has dated a new era in his life from the reading of a book.
Henry David Thoreau (1817 - 1862)