Where Ilford or others states in a papers' Data Sheet an ISO Paper Speed P500 (Unfiltered) followed by Filter Grades and Speed (P) numbers ....
And a Contrast - ISO Range R100 (unfiltered) followed by Filter Grades and Range ( R) numbers ....
What does this information tell you? How is it used?
The ISOP Figure is not important in practice. It states the amount of light (H0.6 in lxs) necessary to gain a density of 0,6logD (ISOP = 1000/H0.6) on the paper. A value of ISOP 500 means, that it requires 2lxs on this paper to gain a density of 0,6logD.
The contrast range stated by ISOR is much more important. It directly states the density range you can copy onto this paper. For convenience, these values are multiplied by 100. If your negative has a density range of 1,0 logD, a Paper with ISOR 100 exactly fits this range. I.e. you will neither loose detail nor will you waste any tonal values.
</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Aggie @ Feb 12 2003, 08:25 PM)</td></tr><tr><td id='QUOTE'>could you put that in blonde speak?</td></tr></table><span class='postcolor'>
Although it will take a while until I reach your level of an "Advanced Member", I nevertheless will give it a try ;-)
If your lab-meter shows a reading of 1 second for the shadows and a reading of 8 Seconds for the highlights, the density range (contrast) of your negative is 8:1. However, Photo Technicians do not calculate with arithmetic ratios, but use logarithms instead. The density range of your negative would be stated as log(8) = 0.9logD. In this case, a paper with an ISOR value of 0,9 x 100 = 90 would have exactly the right gradation. Such a paper would map the shadows of your negative to black and the highlights to white and thus gives you the best separation of in-between densities (i.e. the broadest range of tonal values).
And just to make a few more connections from Thilo's explanation and put it in terms of "stops":
A luminance range of 8:1 or 8x is equivalent to 3 stops. If you had a filter with an exposure factor of 8x, that would also be three stops. In log terms, an ND filter with a density of .3 is 1 stop, and a ND filter with a density of .9 is also three stops--the same three stops as the density range of Thilo's hypothetical image, which had a density range of 0.9 logD, the same units as are used for describing neutral density filters.
This is also the same unit used on a densitometer, so when Ansel Adams says in the context of the discussion of the development test that he looks for a Zone VIII density of 1.2 for printing on a diffusion enlarger, that means 4 stops above the base+fog density. A film with a Dmax of 2.7 would have a maximum range of 9 stops.