San Francisco Text 字體包含 G1、G2、G3 字重的意圖是什麼？

感覺 G1、G2、G3 的粗細略有差別，發布這些小有差別字體用意是什麼呢？

有非常細微的字重差距。如圖是在 Sketch 中將四個字重 (Regular, Regular G1, G2, G3) 同位置疊加，並將混合模式設為 Difference 的效果。

猜測是用以微調灰度。可能 Apple Watch 會在不同界面使用不同明度的背景顏色。分灰度有微差的四檔，可能是用以維持在不同背景下文字的視覺粗度一致性。

應該是為了改善在小字型大小下的可讀性：

The system font was designed specifically for legibility on Apple Watch. At large sizes, the font』s slightly condensed letters are set tight to take up less horizontal space. But at small sizes, they are spaced more loosely and have bigger apertures in glyphs like 『a』 and 『e』 to make these easier to read at a glance. Punctuation is also proportionally larger when the font gets smaller. And as text size changes, Apple Watch dynamically switches between fonts to maintain clarity and legibility at all times.