Thanks a lot Chris, it is a good starting point for a newbie like me. I have some 13x18 enlargements that i like a lot and i will start asap to enlarge it at 20x30. I wanted to find the calculation for the time but i think your process could be useful & also more easier for me.
Originally Posted by Christopher Walrath
Test. Unless you're wealthy. In which case send me some paper!
easiest is open the lens stop. From 35 mm, 3 1/2x5, 5x7, 8x10,11x14, & 16x20 each require one stop more.
Another way is to proportion the area of the two papers. 4 8x10 fit on a 16x20 so you need 4x the light or 2 stops.
1 stop is 2x. 2 stops are 4x, 3 stops are 8x.
If you start changing times rather than light quantity, reciprocity factors come into play and bigger prints will be too light.
So my formula is 8x10 is f11, 11x14 is f8, 16x20 is f5.6, 5x7 is f16
It is also possible to use neutral density filter. B&H sells Roscoe light modifying ones. .3 is one stop. .6 os 2 stops .9 is 3 stops. $6 for 20x24 sheet.
Finally I have an Aristo resistor made for their cold lights that dims to 3.5 stops constantly variable. Use a wall light dimmer for 1/10 the money.
You might find that increasing exposure alone when going from 8x10 to 16x20 is not enough. Sometimes you need a slight bump in contrast with the larger print.
The Darkroom Automation timer looks nice. The Ilford exposure meter is an inexpensive option too. Just calibrate it to a zone on the 8x10 print and then when making the 16x20 you open up the aperture until the meter nulls out.
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It ought to be simple but it isn't. See Sticky Thread at top of this forum - pages of agonising about essentially the same thing. For some reason a mathematical approach just doesn't work - in the event contrast is not quite the same, exposure not quite the same, burn times as proportion of exposure not the same; and even if, as pointed out, a highlight or shadow is just as in the 10 x 8, in the bigger size it doesn't look right any more. And of course there are lots of reasons, especially if you are lengthening the exposure rather than opening up the aperture - reciprocity failure, maybe a little flash effect from the safelight that was meant ot be safe, light spill from the enlarger, light leak through the ceiling, your lovely pack of 20 x 16 is older than your fresh 10 x 8; need I go on?
Originally Posted by jeffreyg
So, I always start again; at least you know what you are after. I start again with my trusty RH Designs Analyser to give me a basic exposure, then put two or three or four 5 x 7s onto key areas (ideally cut from the pack of 20 x 16, but as I use MGIV RC I can usually just use tailor-mades), dev and fix; then maybe a test strip with my Ralph Lambrecht test strip printer to fine tune exposure; then Bob's your uncle, a final print and it's not even supper time yet. It's not really a hassle, after all most of us don't make 20 x 16s of every neg; I may go months without feeling I have a negative worth the treatment, but when I do, it's special.
PS I've done all that midnight oil burning; I'm retired now, so I can do it in the afternoon.