I had wondered the same thing, so I decided to have a quick look around the web for some research on the topic. Turns out, it is damn difficult to find any sort of empirical test that consider BOTH readability AND eye strain. In terms of sheer readability, the research is pretty clear: Black on white always wins, and particularly so with participants with less than perfect vision.
At the same time, however, pure black on white puts a huge strain on our eyes later in the day, as bright white light pours from our monitors. It can be downright jarring to read anything after the sun has gone down, if the text is pure black on a pure white background.
It is also worthy of note that many dyslexic users are sensitive to brightness and very high contrast, which can cause words to blur together (according to uxmovement.com’s article, the source link, however, is broken.)
I think that as long as you adhere to the AA (Contrast ratio of 4.5:1) or AAA (Contrast ratio of 7:1) criterion set by W3.org, you can safely venture into the proverbial and literal gray area.
I found this website to be the quickest and easiest way to calculate the contrast ratio.