I'm trying to achieve very fine line-work in my svg/pdf's. When I use a stroke-width of say 0.5pt, I can see very thin lines in the browser; however, my Princed pdf shows considerably thicker lines (even when zoomed in). I can't seem to get the lines finer than 1pt.

Does Prince have a minimum stroke-width built in? Should I use different units?

Any suggestions are appreciated.

There is no minimum stroke-width, here is a test SVG which demonstrates:

<svg viewBox="0 0 100 100" width="100pt">
<line x1="10" y1="10" x2="10" y2="90" stroke="black" stroke-width="4"/>
<line x1="20" y1="10" x2="20" y2="90" stroke="black" stroke-width="2"/>
<line x1="30" y1="10" x2="30" y2="90" stroke="black" stroke-width="1"/>
<line x1="40" y1="10" x2="40" y2="90" stroke="black" stroke-width="0.5"/>
<line x1="50" y1="10" x2="50" y2="90" stroke="black" stroke-width="0.25"/>
<line x1="60" y1="10" x2="60" y2="90" stroke="black" stroke-width="0.125"/>
<line x1="70" y1="10" x2="70" y2="90" stroke="black" stroke-width="0.0625"/>
</svg>

The stroke width is getting embedded in the PDF exactly as specified, eg. 0.0625pt.