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I have been looking at Mercury in the course of trying to select a typeface for an upcoming book project (my first 'serious' book actually!), and got pretty excited about them offering different grades to offset differences between paper / printing circumstances.
Now to my uninformed eye, the difference between the grades looks very similar to the difference between optical sizes in other typefaces, except that the different grades all share common widths / metrics so as to be interchangeable at any time. – Also, on the H&FJ page that helps in choosing a grade, under the Book section it says, "Book designers who use uncoated stock (and can rely on good inking) typically select Grade Two. Those who routinely run very small type, such as footnotes or captions, should consider adding Grade Three."
Now – wouldn't that effectively mean using the heavier grade in the function of a smaller optical size?
So, can 'grades' indeed be seen as sort of an elegant digital adaptation, or expansion, of the concept of optical sizes – as in, you need a smaller size, you choose a heavier grade (& most likely track it more loosely) – but also, if you need the same size but the print comes out too light, you can choose a heavier grade as well without the layout changing?
Or am I mixing up apples and oranges?
(If this sounds confused, it's probably because I am. :) )