Homebrew PCBs

The conventional method of using photoresist is to install a mask on top the
photoresist.  I'm considering using a two-to-one reducing lens and placing
the mask above the reducing lens.  Since the mask image would be reduced to
half size when it hits the copper clad, the fine detail requirements of the
mask and printer are reduced. For example a 5 mil trance on the PCB would be
printed as a 10 mil trace on the mask (printer transparency).  Is a reducing
lens possible?  Are there any tradeoffs with a reducing method?

 

A setup for a reducing lens -
http://archive.cnx.org/resources/fb001372e360c7bddb6939364bfb77a57420e7dd/5.
15.png