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OK, I think I have down the general idea and it's not too complex: type designers create type to look good at certain sizes. H&FJ have a nice graphic illustrating this by showing the type at different sizes and shading out areas where the type is not intended for use.
I own Brioso Pro, my first purchase of an optical typeface, and the specimen book goes to some length to explain the uses of the various sizes.
As I mentioned in my most recent post, I'm terribly new to the issues of real type and typesetting, and I'm only self-taught, so please forgive me for asking what is likely an astonishingly basic question:
I see typefaces where there is a number after the number of the name, eg, Didot 16, Cycles 24. I am embarrassed to admit that I have wondered for you years what the numbers represented. I can't believe that "font size" never occurred to me! Je m'accuse! However, after I realized what it meant, I read somewhere (this site? a book?) that just because that number represented a size didn't mean that one was bound to that size, so I could have Didot 16 at, say, 11/13 (eleven point type with 13 point leading—in case I used the wrong convention).
So, that's what I don't understand. If Didot 16 is designed for 16pt type, then why set it at anything else? Or, is this a case of the perfect being an enemy of the good? We vary the optical to make sizes we don't have because the designer can't simply design every single point size imaginable?
I'm not even sure how to search for this or what terms to use. If there is a simple explanation, or if someone could point me to a resource or to, I'd be most grateful!