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You're getting a false preview because you're not viewing at 100%. You must view at 100% for an accurate preview.
The change you're seeing is not real, it's an on-screen artifact caused by resampling and blurring of the screen image. The adjustment preview is calculated on the basis of this, for performance reasons.
Tonal adjustments have no effect on "binary" images where you have either/or values. Line art and hand writing fall into this category. A mid-tonal adjustment does nothing to a very light or very dark value. It just appears to do so, because the scaling/softening introduces mid-gray values that aren't there in the original.
To find the best and most efficient way to do what you want, provide a screenshot/example. If screenshot, it must be at 100%.
Thanx for your advice! May I quote your mail, since you didn't post it here? Perhaps it might also be of help to others.
To increase contrast in this case, use Levels and pull the end-points inwards. Don't use the middle slider, it won't do anything, and don't use Curves (unless you move the endpoints).
I guess what you suggest will do the job - at least it sounds good. However, since I am using the German version, I'm not quite sure, what kind of tool you mean by "Levels" ("Ebenen"?), and what you mean by "pulling the end-points inward". A screenshot might help me to identify the tool.