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Light units and Exposure times

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Light units and Exposure times

Postby strackmatt » Tue Feb 06, 2007 7:48 pm

I'm trying to figure out how to over expose a screen (to choke back a little for underbase white screens)with a table that has a timer only. When I was using units I'd add 10 light units or slide a piece of blank film between the table and the screen to block it out a little.

What I'm asking is how do I convert 10 units to time, or how long should I expose for to choke back?

Thanks!
Thanks,
Matt
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Postby andymac » Tue Jul 10, 2007 12:00 pm

there is no accurate conversion of time/units when switching between different exposing systems. You need to run a test on the new exposing table with the timer to determin your best /complete exposure - then once you have that, it's easy to add the equivalent to your '10' from the other integrator controlled system.

divide the overexposure amount by the regular amount to find the multiplier on your old system (say it was 47 units for a regular exposure, 57 for over. you multiplier is 57 divided by 47 =1.21

Now use that on your new machine's time for a regular exposure which you have determined through testing using a step wedge exposure or an exposure calculator. (say it was 5 and 1/2 minutes, convert to seconds = 330 secs x 1.21 = 399 is your new overexposed time, equivalent to your old increase using units. ) divide 399 by 60 seconds and the time in minutes is 6 min 39 seconds :idea:
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Postby Jeremy » Tue Jul 10, 2007 9:38 pm

Hmm... could you explain why you would want to choke the screen? This sounds like a technique I'd like to hear more about. I think I get it, but... :D
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Postby andymac » Fri Jul 13, 2007 4:03 am

I don't know why he's doing it, but....

Choke and spread are two 'oldfasioned' ways of making fim positives smaller (choke) or bigger (spread) using (typically) a process camera - they would use this to alter films slightly to create traps or eliminate edge by under or over exposing contact film of an image.

so what our correspondent is doing is choking (making smaller) a stencil for his underbase white that is slightly smaller than his existing print. do this by overexposing all the films sandwiched together - the light creeps under the edge and closes in the image - same as if you overexpose type or halftone dots and they close in.

make sense?

they have these amazing 'computers' now with 'programs' that actually do all this for you - sometimes- and then you push a button - yes, I know it sounds too good to be true, but we live in amazing times - and the film comes out of the machine ready to burn with an underbase created from the art and shrunk exactly like you need.

analog vs digital......go dinosaurs!
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