Light text on a dark background vs. dark text on a light background: which allows the reader to keep focusing for longer?

The most fundamental fundamental of good typography for long text is that the type should be “invisible” to the reader, so there is nothing to interfere with the communication of information. From that, we get the principles that serif faces and lightweight sans are better long text than regular sans, and dark on light tends to be preferable to light on dark. There is a short summary of this generally accepted typographic wisdom here. You really can’t divorce readability from comprehension, and therefore retention.

That said, I know several reasons for lack of retention that have nothing to do with typography, and I’ve also seen some downright idiotic research in this area (like the guy who cut text columns down the middle and separated them by 1/4 inch because it “forced the reader to concentrate more in order to follow the text”). So your experience in this case may be related to the type, and it may not.

Sticking with typography, though, the bottom line is that what is comfortable to read is more likely to be absorbed than what is not.

There’s a raft of research in the optometric field, particularly vision therapy, pointing to a connection between poor eye coordination (“convergence insufficiency” in the jargon) and inattentiveness, lack of retention, even a dislike of reading generally. (Often misdiagnosed as dyslexia or ADHD.) The effect of the various forms of convergence insufficiency is a kind of eyestrain that makes reading uncomfortable. The same is true of people who need corrective lenses to read but don’t wear them.

It’s not much of a stretch to conclude that anything about the typography of a long text passage which is less-than-ideal is likely to have a negative effect on attention and retention.

In general, the biggest source of discomfort when reading long text passages is excessive contrast. The ni plus ultra of this is taking a book that’s printed on bright white paper and trying to read it in direct sunlight.

When looking at a generally dark field (light-on-dark text, for example), the pupil of the eye widens. Without getting into the technical details, wider pupil == less sharp focus, a slight fuzzing out at the edges of the type. That’s just how optics work. This is also why it’s so painful to read in dim light. The pupils are at maximum dilation, which means that focus is at its worst.

Looking at a light page, the pupil narrows. Narrow pupil == sharper focus (which incidentally increases the edge contrast of objects in the visual field). From this, you might conclude that sans type, being of chunkier build than its serifed cousins, reads better reversed out of a dark background than a serif face, and you would be right.

Helvetica and similar sans faces create strong contrast with their background, and are uncomfortable to read when rendered black on white or white on black, unless you use the lightest weight. When I’m setting text in a sans, I’ll use the lightest weight available and/or something like a grey

Traditional serif text faces such as Caslon, Jenson and Garamond have less visual contrast printed on a light substrate, so are more comfortable to read. Slab serifs like Egyptian Slate are readable only in lighter weights. A Didone or Bodoni, with its strongly contrasting thick and thin strokes, tends to be too contrasty for comfortable reading.

Focus problems can be subtle. It doesn’t have to be a question of light-on-dark. The eye focuses red and green light differently, and if you really want to give yourself a headache, convert a few pages of text to red on green and try to read them. It hurts!