This portion, imaged by the fovea portion of the retina, is focused on an area about the size of a single word at 22 inches away. Outside of the fovea, we have very little resolution but exquisite sensitivity to brightness changes. This allowed our primitive forebears to quickly detect a predator that might be attacking. This substantially slows down reading speeds, which is one reason that reading on a screen is less pleasant than reading a printed book. Another issue is contrast .
Typical screens are perhaps half of that.
A crucial issue is resolution . Some computer screens provide color, but the portable ones usually do not. The size, weight , and cost of notebook computers and electronic books are impressive, but still not competitive with the good old print version. Finally, there is the issue of the available software , by which I mean the enormous installed base of print books. There are 50,000 new print books published each year in the United States and millions of books in circulation.