Hi boys & girls.
I'm new here & checked out the original question about print exposures & formulas etc....
then came to this last page to see how long it went on for. mmmmm?
So, just in case no-one has mentioned this method. Read on.
I haven't done any printing for a couple of years now. But I have always used the guide told to me by my darkroom manager when I was a young lad starting in BW photo-printing.
Based on knowing the correct exposure for an SS print (e.g. 5x4 neg to 5x4 print).
If you measure the length of the print for a 10x8 - 10 inches (twice as long as 5 inches).
You just open up the lens 1 f-stop. Giving twice the light strength.
Then with the same neg do a 20x16 print (20 inches long = 4 x the original length) you open up 3 stops from original exposure.
For example: SS exposure of 5x4 neg to 5x4 print is, say 10 sec @ f32,
So 5x4 to 10x8 = 10 sec @ f22, & @ 15 inches long = F16 & 20 inches long = f11 @ 10 sec.
When you run out of f-stops, you then increase the time comparable to f-stops to suit the magnification.(open up 1 stop for every 5 inches) (that's what she said?).
This method flies in the face of the Inverse Square Law. However, it does work. I have used it since 1976 & it is usually on the button or within a 1/4 of a stop on massive blowups. But it is a very easy quick guide to save time. It also takes the 'Brain Damage' out of the job.
BTW. 5x4 to 20x16 I open up 3 stops (equal to 8 times the illumination. Whereas Inverse Square Law would say the area is 16 times as big as the original 5x4. Thus indicating a 4 stop difference. So anyone who would like to try that little experiment & see how it pans out, please do. sometimes it can be that simple. Honest.
Like a 10x8 neg to 15x12 print in this instance would be either 10 sec @ f22.1/2 or 15 sec @ f32 (same thing).
And so on throughout all the percentage ranges.
So a 10x8 neg printing to 5x4 (50% print size) would need to shut down 1 stop to f45 ( or 5 sec @ f32).
Remember though. Start with a really good neg to form your basis. Then you can up or down the corrective exposures for heavy or thin negs accordingly.
I think i've been quite thorough there.