I have a project in which optically corrected stems inthe heaviest fonts have values very different from the standard stems in the font. For example, the internal horizontal stems of a and e are 48 units smaller than the lightest horizontal stem defined in font info. But these stems are close to the standard stem weights in the light and regular masters. Do I need to add outlier stem values to all masters to make these values interpolate? Do I not hint these stems? Do I manually assign the smallest horizontal value already defined even though it’s much too heavy?
I suppose you are referring to PS Hinting. You do not need to add extra stem values. If they are in the same ball park, that will do.
Try to have only one stem value. Perhaps two if you have something horizontal that is very different, like the thickness of a serif. If you have three, I would bet money that you can do without one of them. If you have four, I would bet a lot of money. And I am not the gambling type.
Keep in mind that whatever gets different stem values assigned, may be rendered with a different amount of pixels in a low-res situation. So, only introduce an extra stem if it is bearable to you that one stem is rendered with 1 pixel while the other stem is rendered with 2 pixels at a certain threshold PPM size.
I’m referring to TT hinting.
I see. You can have more stems in TT hinting, but stems that are visually the same (and this is the case here if I understand it right) usually should receive the same stem assignment. So I wouldn’t introduce extra stems unless really necessary.