K Michael , Sep 24, 2002; 11:09 p.m.
I'm confused about paper ISO and how to calculate the new exposure
time at a different contrast. Suppose at at grade 3, my paper ISO is
110 with an exposure of 15 seconds at f/11. Now I want to make the
same print in a paper grade where the paper ISO is 40. What is the
mathamatical formular for calculating this? I've been making new test
strips for the new contrast grade but find this time consuming.
Art Haykin 
, Sep 25, 2002; 03:08 a.m.
To begin, no paper I know of even approaches an ISO of 110. Try 1, 2, 3, or less. Second, I assume you are using graded paper rather than VC type filters. Paper speed is NOT necessarily linked to grade, though speeds MAY vary from grade to grade, brand to brand, batch to batch, and with age, etc.If you have a selection of grades, I suggest you run some speed calibration tests, and mark the boxes. That, or get aninexpensive enlarging meter, and calibrate it. Lastly, most printers I know make test strips. I did, for 60 years.
Marc Leest , Sep 25, 2002; 04:12 a.m.
Well the iso of the paper is ISO-R and indicates contrast grade. (that another standard !). Grade 2 is +/- ISO-R 100.
Use this calculation (estimation) to find the desired iso-r grade:
Use a comparator to determine the exposure time of the shadows (T1)
then determine the exposure time of the highlights (T2).
Iso-R = (round) 100 * log10(T1/T2). If eg. T1 = 60s and T2=5 iso-r=107
hope this helps.
Art Haykin , Sep 25, 2002; 05:34 a.m.
Pete Andrews , Sep 25, 2002; 09:23 a.m.
Well, it SHOULD work out to 110/40 * 15 seconds, with your figures, since an ISO rating is linear. This is 41.5 seconds. However as Art succinctly puts it, the theory and practise are two different things.
<br>Use 40 seconds as a starting point for your exposures with the slower paper, or better yet, open up to f/8 and test around 20 seconds.<p>The formula is: New exposure time = old exposure time * old ISO speed/new ISO speed.
Ken Burns , Sep 25, 2002; 12:33 p.m.
I have found that paper speed ratings are, at best, only very approximate and vary so much from batch to batch that they are mostly worthless. If you wanted to calibrate paper batches yourself to a particular aim point, you might be successful, but that is probably only practical if you purchase paper in large quantities. Of course paper speeds vary tremendously as the paper ages, so you would need to either use up the paper quickly, recalibrate regularly, and/or keep the unused paper frozen. Test strips might be easier in the long run.
Chris Waller , Sep 26, 2002; 07:23 a.m.
The ISO speed of paper is based on a different scale to that of film. But if you work out the 'film ISO' speed of paper it comes out about 3.
