Can’t soft proof a duotone in Photoshop. Have to convert to RGB to soft proof; however, the color changes in subtle ways.
One way to compare the images is by checking their histograms.
The duotone image is just a little darker on average (mean 106.97 compared to mean 112.53), and it clearly has pixels in the darkest bins, including a spike of 0% black over on the left edge of its histogram. The RGB image has no black pixels until you reach level 11, about a 4% gray.
You can use Curves adjustment layers to compare the dynamic ranges of the two versions of the image. Option-drag on the black point or the white point in the Curves dialog to see where in the image the darkest and lightest values are found. The duotone reveals solid black (0%) in a few places. The RGB version has no solid blacks. In the light tones, the two are practically the same.
You can also highlight differences between two images by putting one in a layer above the other and applying Difference blend mode to the top image. I copy the duotone and paste it over the RGB image and then set the duotone layer to Difference mode. I exaggerate the differences with a Curves adjustment layer, dragging the white point slider all the way over to the left as far as it will go (to output: 255, input: 4). You can see a lot of dark pixel noise from minor differences that have been exaggerated, and black areas will stand out as shadowy shapes.
But the real proof is in the printing. I’m going to print the duotone and the RGB on an Epson 9900 and compare. I’ll let Photoshop handle the colors, and print to the profile of the paper I’m using, Hahnemuehle Photo Rag.
Now I’m looking at the two prints in a viewing booth. The differences are subtle, but the duotone does seem to have a little more dynamic range. This confirms the various comparisons I already made. In some areas, the increased dynamic range results in slightly better discrimination of details. There also seems to be a very subtle difference in tonality between the two images. The RGB may be just little warmer–but that could be an effect of the narrower dynamic range. Color differences are so small as to be practically impossible to detect with any certainty with the naked eye (or the eye with eyeglasses, in my case).
The duotone image I’ve been using for these experiments was one of 17 images from the series Deadpan, or, the Holy Toast, generated by 3D software (Pixar’s Renderman) and intended to mimic the appearance of old photography on paper. Applying the duotone was the final step in that imitation. Arguably, the diminished dynamic range of the RGB conversion might work better towards that goal, and I can certainly use a Curves adjustment layer to compensate for the changes, if I want. That fact that the standard deviation for both images is practically identical suggests that the conversion process was more or less linear–it just squeezed the dynamic range. However, I find the greater dynamic range desirable. It may be simpler just to print the duotones as duotones and let Photoshop handle the conversion to numeric values for the printer. That appears to be amore accurate printing workflow.
How important is soft proofing in this case? For an image that is nearly grayscale, not as important as it would be for an image in color. The monitor image is “in the ballpark” almost as much as the soft-proofed RGB image. Since I do the printing myself, on an ink jet printer, and don’t have to worry about offset printing with custom colors for a true duotone, I have less need of the soft proofing functions. All the images have a common origin, so whatever corrections apply to one will apply to the others. And I can probably do effective soft proofing with an RGB image with a corrected dynamic range if I really think it necessary.
Conclusion: in this instance I like having the improved detail offered by a wider dynamic range, and I like to have a truly dense black in the image, too. Soft-proofing isn’t enough of an issue to step out of duotone mode: that’s how I’ll print the new edition of Deadpan, which I first printed in 1995 in small format on a Fujix printer. I’m looking forward to printing large images, with details visible from a distance. Even at the original size, the Epson 9900 delivers better image quality–printers have come a long way in 15 years.