Optical size

Adjusting a letterform for the size it will be seen at: sturdier when small, finer when large. Type that ignores this looks wrong without saying why.

Type was never one drawing at every size. Metal faces were cut differently for six point and sixty: sturdier strokes and open counters to survive small settings, finer contrast to reward large ones. Optical sizing is that judgement returned, one design flexing with its size.

Modern variable fonts carry an optical axis that does the flexing automatically, and it changes what a family can do: captions hold open, display cuts turn fine, all from one file. Faces without it hide the compromise in the middle, drawn for no size in particular, slightly wrong at all of them.