I recently stumbled upon Faber-Castell’s Ecco Pigment Pens. My favourite is 0.05 . Just like the Pitt Artist Pens, they are waterproof and have maximum lightfastness.
It is incredible how small you can draw with the 0.05 – that is why I like the pen so much. In the following pics I have added some coins for size comparison. (coins: 2 Czech Koruna, 20 pence, one dime & one cent)