
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 noone 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 photoprinting.
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 fstop. 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 fstops, you then increase the time comparable to fstops 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.
Happy printing.
Jim