I have had this question for a while: Can you use a second correction curve as a way to fine tune the the first one if the latter didn't quite give a good enough linearity in the print? My intuition was that you should be able to. A couple of times I tried this, the results were not consistent. In real application, there is always some noise introduced because of print-to-print variations in the process, particularly earlier in the development. So it was not immediately clear to me where the problem was. To figure it out, I decided to do a simulation of the process using Photoshop and ChartThrob. (I am sparing the details here unless someone wants to know the methodology.) From this I was able to convince myself that if the first curve is somehow not accurate, one could derive a second curve using the first curve as a starting point. The two curves can then be used in conjunction to get the required linearity in the final print. However, there is a catch. The second curve must be used BEFORE the first curve, not AFTER as one might have tendency to do in a Photoshop application. I tried many different first curves, from being very close to the theoretical to completely out-of-whack and arrived at the same result. In retrospect, thinking some more why the position of the curve matters, it finally downed to me that it made perfect sense. The first curve is for correction of the ensuing process which can be seen as lumping together of three steps which are a) inversion to negative and flipping, b) addition of colorized layer, and finally c) making of the print. In next iteration the process can be seen a combination with the first curve as the first step and the rest same as before. This would necessitate that the second curve be placed before the second curve. What is the point of all this? In many processes like salt print, density change is extremely gradual in the shadows, with the toe part of the characteristic S-curve sometimes extending to almost half of the range, due to a strong self-masking effect. This squeezes the zone of usable data to only a part of full 0-100%B range. There is a potential for lack of accuracy in the resulting correction curve because of noisiness in the data as well as difficulty of ChartThrob script to approximate the initial part of the curve where the slope is extreme, not to mention the funky way Photoshop “smooths” the line in Curves layer. For that reason, I have started to use a generic “starter” curve suited to the process at hand, then calculating the second curve from the test print made with this starter curve. Finally, the two curves are used together in the proper order to make the real print. FWIW.... :Niranjan.