Exposing film at a higher ISO rating than normal, and using the increased development times recommended for the higher ISO speed, is by definition N+ ("push") processing. Push development doesn't really increase the speed of the film as Adams measured it (approximate density of .1 over base film and fog for a Zone I exposure); it merely increases contrast of the negative. This is because increased development has little effect on the shadow areas (Zones I-III) and a much greater effect on the highlights (Zone VII and above). So I believe that you are barking up the wrong tree if you want N-1.(posted 8311 days ago)To achieve N-1, Adams would usually cut the normal ISO speed by 1/2 (increasing exposure by 1 stop) and cut development by about 15-20%. To exactly determine the processing required for N-1 development and exposure, here is a procedure that can be used:
First, expose an image of a gray card (Zone V) by giving the film 3 stops more than the meter reading. This will give you a Zone VIII image on your negative. (If you know that your meter is not calibrated to 18% gray, then adjust accordingly.) You should develop this roll of film normally (N). Then take another exposure (on another roll or another sheet film) of the gray card and give the film 4 stops more than the meter reading. This will be your N-1 exposure (Zone IX that you will “pull” back to Zone VIII). Develop the N-1 roll (or sheet) about 20% less than the first roll (N). For the N-1 exposure rate the film a bit lower in ISO speed (1 full stop from normal is recommended).
Print the Zone VIII image on the first roll (N film development) in your enlarger. Experiment with different print exposure times until it comes out as Zone VIII, a slightly off-white with a hint of texture (Adams’ or other books will show you what Zone VIII should look like). Note the enlarger exposure time and print development time carefully.
Then print the image from the N-1 roll (that was given 1 stop more film exposure and 20% less development than the N image) using the exact same enlarger exposure time and same print development time as the first print. Compare the 2 prints. If the second print (N-1) is lighter than the first print (N), reduce your film development time for N-1 and try again. If the N-1 print image is darker the N print, increase your film development time. When the prints match in Zone VIII values, you have “pulled” the negative highlights by one Zone (IX to VIII) and now you have your N-1 film development time.
Conducting your own tests to determine N-x processing is important because your enlarger light source (condenser, dichro, cold-light, etc.) has an impact on the final print contrast you are trying to achieve. Also, your personal film development technique (agitation, total solution amount, etc.) effects negative contrast.