David Lemon from Adobe writes in his tutorial Basic Type 1 Hinting about the stem weights:
"Another font-wide behavior is the stem weights provided for the font. It’s generally best to enter two values for vertical stems and one or two for horizontal, listing the values in ascending order. In a standard Latin font, the values should represent the optimal weight of the normal lowercase and capital stems (hence the two values). These values - especially the first vertical value - are used for a number of rasterization purposes, so it’s important to make them as representative as possible. Two of the most obvious uses include letting ATM know when to turn off synthetic emboldening in a bold design, and serving as target values, to ensure consistent stem weights at low resolutions. In a design where the stems vary slightly in weight, fine-tuning these target values can make a significant difference."
But that is a try-and-error-solution. I would like to know, how I can find the optimal standard stem values in a more systematic way. And I have recognized, that Adobe (for example) does not use three or four values in all cases. Bold or black fonts, in which the contrast is stronger, seem to need more values. In my opinion the only thing in PostScript hinting, which is hard to do, is finding the optimal standard stems, especially because the output of the autohinting script contained in the Adobe Font Development Kit for OpenType (AFDKO) is extremely good. There is another script (setsnap.pl) contained in the AFDKO (coded by Ken Lunde in Perl), but the output was not helpful (even not with the non standard options). Unfortunately there is no tutorial, how to make the optimal use of the script (for the case, that it is useful and I am too stupid).
So my request (because I bang my head against the wall):
Please teach me! I would like to know a systematic way, how to find the optimal standard stem weights and the optimal number of standard stems. These values are extremely important for PostScript hinting. Even if you do not define individual hints for the glyphs, the values of the standard stems help the rasterizer. (Just make a test: Generate a font file, in which the individual hints are removed only. And generate a second file, in which you additionally have removed the standard stem values. Compare them. If the standard stem values were well chosen, as in Adobe fonts for example, you can see, that the font, in which standard stem values exist, looks much better on screen.)