I use preflashing quite often to reduce contrast - I happen to have a 2-1/4 enlarger next to my 5x7 so I use the 2-1/4 enlarger as the light source when I print large format - but any controllable, repeatable, light source will do - (I have a friend (Steve Sherman) who uses an old hanging bullet safelight with on old enlarging lens attached to control the light.)
Originally Posted by htmlguru4242
My procedure is as follows - To determine the flashing time I place the enlarger at a fixed height and aperature and make a test strip at say 2 second intervals and develop it and dry it - what you want to find is the time that creates the first hint of tonality above paper base - then back off your exposure slightly so that the flashing creates no percitable tone - this brings the paper to just below it's threshold so that any additional exposure during printing will produce tonality immediately. You would naturally have to run this test for each paper you use as the flashing time may vary. But once you determine you papers threshold time - you've got it! During printing I will will set my flashing source to it's predetermined height and aperature (which for my paper is 27" and f16 @10 seconds), expose the paper and then put the paper under the other enlarger and make my negative exposure. As I prefer graded paper, this has become an invaluable tool in my printing procedure when contrasts reduction is needed. I sometimes flash to less than threshold when minor adjustments are needed. I hope this helps you.