How thin can/should a hairline weight be (on 1000 upm)? When is too thin?
Crazy questions, please have mercy!
The stem of the upper case 'i' in FF Meta Hairline is 10 em (is that the correct usage?) wide on a 1000 upm grid. I do not imagine that you would want to go much below that.
I'm working on an ultralight, and the stem of the capital I is 18 thick. I'm not going to make it any thinner ;-)
Of course this depends (at minimum) on how big the resulting font will be set.
Luc (de)Groot has done one with 1 unit. I think it is visible at 1 Meter, or was it 10?
What Craig said.
BTW, at 1 unit, can you even make normal curves?!
I think there were curves, but am not exactly sure. I designed my Agilita Hairline with 4 units which was extremely difficult, especially in the Italics. For optical compensation i went down to 3 units. And that was the end of affairs, i found. The next weight UltraThin is four times as thick (16) :-)