The D5 has three position light illumination: White, High, Low. I'm printing B/W 4X5 film. I changed the head because the original (to me) was simply not working any longer. Gels were falling out, filter levers were sticking; it was simply not working and too much for me to deal with. The new/rebuilt head works flawlessly, BUT... where I had always used High light output I now just use Low and base time's are slightly reduced at the same f-Stop. My question is: Is the selection of High -V- Low arbitrary and linked to the negative in hand or do you use one all the time? Which one?