Arial and Verdana.
Or a pixel font.

Because I suspect that’s not the answer you were hoping for, I have heard good things about the RE families from Webtype.com

What ppem size(s) are we talking about?

hhp

What Hrant said: how small a pixel size do you need? 9, 10, 11, 12?

There are other options besides Arial and Verdana, but how many depends on size. For our WebINK web fonts, we have direct px size ratings.

To the best of my knowledge Tahoma and Trebuchet were designed for screen usage. For positive confirmation I’d ask Kevin Larson.

However, beyond ~300 PPI at a distance of ~30 cm, resolution doesn’t matter. With certain exceptions — such as a crappy ATM or giant airport sign — we are moving away from needing to concern ourselves with screen resolution as a factor (and eventually hinting and rasterization).

This has been discussed at great length here, but I do not have any specific threads ready to mind. You can read more about it in an article about Apple Retina Displays from Jonesblog, as well as:

Curcio, C. A., Sloan, K. R., Kalina, R. E. & Hendrickson, A. E. (1990). Human photoreceptor topography. Journal of Comparative Neurology, 292(40), 497-523. doi>10.1002/cne.902920402

Bernard, M., Liao, C. H. & Mills, M., (2001). The effects of font type and size on the legibility and reading time of online text by older adults. Conference on Human Factors in Computing Systems. New York, USA. doi>10.1145/634067.634173

You’ll need database access to get at those last two, but I have had great success simply contacting the primary authour and requesting an electronic copy of the article in the hopes it will you assist you in your research, and you will be able to cite them (academics like to brag about how many times they have been cited).

> beyond ~300 PPI at a distance of ~30 cm, resolution doesn’t matter.

That makes no sense to me.
It makes more sense that those studies are all flawed (which is sadly typical).

hhp

> > beyond ~300 PPI at a distance of ~30 cm, resolution doesn’t matter.

If you were to make that "...doesn't matter for legibility," then I might agree. I'm guessing that's what you meant.

Hrant, why do you expect resolution to still affect legibility under such circumstances? Let's say 300 ppi but at a distance of 60 cm, so typical book-reading or tablet e-reading distance.

Good question, but do we mean legibility or readability?
Heck, the former does fine at double-digit resolutions!
Concerning the latter, since the difference (at the 30cm,
12" typical reading distance for a printed book) between
1200dpi and 2400dpi is noticeable*, but between 2400dpi
and 3600dpi isn't, I'd say at least 2000dpi. I'd estimate
that good** onscreen anti-aliasing halves the resolution
requirement, so around 1000 ppi.

* And anything that can be seen
consciously must affect reading.

** Which BTW is not the iOS/OSX stuff. :-(

hhp

Try my Cabin font from the Google Webfont directory.
It has been hand-hinted recently, and renders beautifully as small as 12 pixels under WindowsXP.

Win Xp's default settings?

Webtype has developed their "RE" series for small sizes. Here they are.

> Win Xp's default settings?
Yep

> Webtype

In a reciprocation of candor: I'm sorry, but that just
doesn't cut it on more than half of the world's computers.

hhp

Hrant, they are true type hinted; what isn't working for you?

Hmmm... I'd say it's the... how you say... rendering.

To be fair, I certainly appreciate the amount of work needed
at arrive at this point - it's something beyond my own abilities.
It's just that I'm pretty sure I believe it's not worth it; and I have
to assume -just for the sake of my own sanity perhaps- that an
actually satisfying solution is still achievable even in this anti-
reading age. Whether it's economically viable is another issue.

hhp

Hrant, on my old Windows machine (Windows XP, Internet Explorer 8, flat screen), the rendering is significantly darker (and better) than what you show. And still darker on my Mac. So I don't know what is going on. But I'd better let others discuss this, as I don't know that much about it.

Not that it solves much, but could you post a screengrab?

hhp

BTW, I just checked the OSX rendering* and
it's certainly darker, but it's also a bit fuzzier.

* Firefox 3.5.3 - is that too old?

hhp

I did a screen grab on the Windows machine, and saved it as a .gif as you did, and then it looked similar to yours. But the original on my screen looked better. So I don't know if creating the .gif did something to the rendering. I'm looking on the Mac using Firefox 8.0.1.

PT Sans Caption can resist 9 pixels WindowsXP Standard Rendering.