I’ll admit it’s not the best phrasing (and I’m definitely not an ophthalmologist, so I’m playing fast and loose here). Your pupils dilate to let in more light because there’s less light overall when the background is black. This causes your lens to distort slightly, thus making the edges of the white text appear to be fuzzy or bleed into surrounding characters. I couldn’t find a scientific source on this, but a couple articles called it the “Halation effect”. Over time, this creates eye strain, and it’s apparently worse for users who have astigmatism. Of course, if the text is sufficiently large enough, this effect should be at least somewhat mitigated. Generally, at least in my use cases, it’s not large enough. Although the more I think about it, I’m rarely really staring at white on black. It’s usually more like various colors against a very dark navy. I should probably just rest my eyes more...
>[...] This causes your lens to distort slightly, thus making the edges of the white text appear to be fuzzy or bleed into surrounding characters
The effect you're describing is correct, but the mechanism isn't. Aperture (pupil) size directly affects how focused the image is. The smaller the aperture, the sharper the image is. As an extreme example, if your aperture is a single point (a pinhole camera), the image can be focused without a lens.
> Articles describing this seems to say it's caused by the film medium, rather than being an optical effect.
Halation in film is caused by reflection from the back of the film. Most of them have an anti-halation layer to reduce this effect. I don't see any reason why one's eye wouldn't also be vulnerable to this effect (and probably has some biological mechanism to prevent light from being reflected out of the eye. Cats sure don't have this, though.)
> Articles describing this seems to say it's caused by the film medium, rather than being an optical effect.
Yeah, I noticed that. Searching the term directly gave me results related to film, but when I used other terms describing the effect they mentioned it in a different context. Makes me wonder if someone is just confused.
To understand the difference between a small pupil and a large one, it's useful to understand how "aperture" settings work on a camera: https://en.wikipedia.org/wiki/Aperture
Specifically, bright light results in a small pupil which allows for better focus. And low light results in a large pupil which reduces focus.
