This is very interesting info, thanks guys.
Testing a current day HP laser printer at 600dpi and 1200dpi did not show
appreciable resolution increase between 600 and 1200. Traces 5mil to 8mil were more gaussian in appearance instead of a desired sharp edge. I envisioned using 35mm film enlarger in
reverse to reduce the mask size but it occurs to me that 35mm is the largest
possible board size.
This guy experimented with a microscope for reduction. Not feasible for PCBs but neat. https://www.allaboutcircuits.com/projects/diy-pcb-photolithography-microfeature-fabrication/