Many of the adjustments that are made for image tones relate directly to how they will print as halftone dots. As seen below, for lighter tones we are speaking of increasingly smaller black dots, and for darker tones increasingly smaller white dots. In print production there are certain limitations and tolerances that will effect how these dots will appear.
For conventional printing, the ink that is being used (such as black) will absorb into the paper which can result in having the image swell in size. The printed black dots become larger ("dot gain"), and the printed areas behind the "white dots" become wider. This too can happen with digital printing where toner (rather than ink) lays on, and is baked onto, the paper's surface. This layer of toner has a thickness to it which too can effect the sizes of the dots.
There are many considerations for anticipating how much "gain" to expect in production. Much of this goes beyond what is needed at this point for this discussion. One consideration though that should be mentioned is noting the stock selection. The paper being used can have a huge effect on what will happen once ink or toner is applied. Generally speaking, coated stocks are better for holding printed detail than uncoated papers.
For refining tones with taking dot gain into consideration, would like to first consider the darkest areas of the image. In the example above, at some point the white dots will close up and no longer be visible. Much of what is to the right will print as 100% black (without the small white dots). In addition to this, many of the other darker tones, and the mid tones, may print darker than anticipated.
For the lighter tones of an image, the opposite can happen. At some point the dots become too small to print. Rather than things printing darker (as mentioned previously for the mid and dark tones), some of the very light areas may print lighter. If tonal detail is found in these lightest areas of an image, this may be lost or diminished once printed.
What to do?
Though the description of the problem may seem confusing, the solution really is not that bad. One way of looking at it is that rather than having a color palette (in this instance of grays) that run from pure white (0%) to full black (100%), you would have a slightly adjusted palette (with different values).
For example, if printing on coated paper, it could be that your adjusted palette of grays goes from 4% to 96%. When adjusting tones for your image for highlights, all tonal detail should be at least 4% or higher (anything less may not print and appear as pure white). For darkest tones, these can be adjusted so that they are at most 96% (anything greater will print as full black; the small "white" dots not being visible). Keeping all the tonal detail within 4-96% should do the trick. If printing on uncoated stock, different values may be needed - such as 7-92%. (The print shop may have some suggestions for what values would be best for the equipment and stock being used for the job.)
Anther consideration is with all those tones between the extreme ones. If an image is mostly dark tones, rather than running everything up close to the edge (on the borderline of being black), maybe they can be lightened with the darkest areas of the image being set much lower, such as in the lower 80%s. For all images, it may be good to anticipate that all mid to dark toned areas will appear a bit darker once printed.
Once you get past all of the technical details, when it comes to adjusting tones, it really is not that bad. It becomes less of a value-specific process and becomes more intuitive. You soon develop "an eye" for seeing tones in a different way - expecting to see mid to dark toned areas as lighter shades gray; highlights as darker shades of gray; white as a very light gray (of a certain value); and black as a dark gray (of a certain value). This is not to say that some details cannot go beyond these limits. Much of this is a judgment call. In some instances highlights may be small and in the middle of the shot, and full white may be fine. At other times, some of the darkest points may be details that are linear in nature and full percentage of black may be fine. Decisions on all of this can become natural and will become less about the numbers and more about the aesthetics of what you are seeing - with adjusted eyes.~~~