I say she was serious because photographers (fellows like Jacques Malignon and Arthur Elgort) could never get her to smile. Not even a small smile. But then, her determined expression is what makes her so beautiful.
Irving Penn got her to grin in one of his photos, but that photo is a bit too risqué for this blog.
But this one isn't.
Neither is this one.
The photo above is from the cover of Time magazine's August 30, 1982 issue, the cover story being not about Ms. Foster but about the emerging trend of muscular strength as a feminine beauty ideal. See, she is serious!
Want to know what that article said? Read the archived version here.