Homebrew PCBs

Assuming your film wants a "hold at 8 on a 21" (riston films do), you
want the results of your test to show that step 8 had film, but step 9
didn't.  More exposure = higher steps.  So if you're holding step 4 at
340 seconds, and you want to hold step 8, you have to expose for 4x the
time, or 1360 seconds (~22 minutes).

You can get a much better idea of what's happening if you include a
print of some stripes under the wedge; if you then overexpose you'll see
a range of "dark" steps, followed by some striped steps, followed by
some "light" steps.  The number of striped sets tells you your effective
constrast ratio, and you want to adjust your exposure so that range of
steps is centered at *step zero* (step zero is "fully transparent" so
this is what your board will eventually see).  So if you overexpose, and
see stripes on steps 4-8, you want to reduce your exposure by 6 steps.

You do not want the *held* step to be near step zero.  That means you're
just barely exposing it enough.  You don't want "just barely" you want
"just right".

> I think I might have to write this up for the PCBWiki :)

I've written many posts about how to use a step wedge, so check the
archives first and make sure you capture it all :-)