Gene Nocon's book on the darkroom has the formula. I can't seem to put my
hands on it right now. I have it scribbled on a small calculator I use in
the DR for such things.
However, I use a probe, as I have often posted. With the probe, at any
height, once the 'green' light is found, the print times are the same for
any size.
Oh, here's the formula:
T2= (L2/L1) [squared] x T1 or the time of the second exposure at another
height, equals the length of the second exposure's long side, divided by the
length of the first print -- squared -- times the time of the first print.
Steve Shapiro
----- Original Message -----
From: "Elgenper" <elgenper@xxxxxxxxxxxx>
To: "List for Photo/Imaging Educators - Professionals - Students"
<photoforum@xxxxxxxxxxxxxxxxxxxxxx>
Sent: Friday, June 24, 2005 1:52 AM
Subject: Re: Any math to corelate B&W printing times to print size?
Me again... (Suppose we are in the same time zone, and our American
friends are asleep now).
If you are doing this with head height as a variable, it will be more
complicated than the inverse square law. The reason is that you change
the distance between negative and lens when refocussing, and thus the
top angle of the light cone will change too. This effect is negligible
when using a camera at normal distances, but when enlarging, you´re
within the "closeup" range, and will have to compensate, just as when
using closeup tubes or a bellows unit.
I recall from my darkroom days that the error from using the square law
was quite observable. What I did was computing the paper surface that
should be exposed (assuming identical cropping). So if I had the
correct time for a 4 x 5 print, I had to use four times as long for an
8 x 10, since the area is four times greater (but the head height will
be less than doubled to get the same cropping).
And then, of course, there is the reciprocity effect.... So, to get
the exact exposure I still had to make a new test strip; the "surface
rule" was a great help in seeing where I should start testing.
Per Öfverbeck
http://foto.ofverbeck.se
2005-06-24 kl. 10.00 skrev kpp@xxxxxxxxxx:
just a-wondering...
does the law of light intensity dropping i proportion to the square of the
distance apply to B&W enlargers?
i mean if a print with the head at X height will I have to inrease time 4
times for a 2X printing height? will opening the aperture 2 stops be the
same? (that is for approximations and quriosity only)
are there any details not obvious in this issue?
thanks, kostas
