You asked
Best portrait lens for the sony a9?
For portraits on the a9, I'd start with the Sony FE 50-150mm f/2 GM. It's fast enough for soft backgrounds and sharp enough for close-up faces. If you want more reach, the Sony FE 70-200mm f/2.8 GM OSS is the classic pick, and the Tamron 70-180mm f/2.8 Di III VXD G2 is a lighter, cheaper route to a similar look. The Samyang AF 60-180mm f/2.8 is another option if you want a manual-feel lens that covers the same range.
Before you spend on glass, though, read portraits. The lens matters a lot, but window light and a little turn of the subject's body will do more than any lens. Try light beats gear first, then pick the lens that suits how close you like to stand.
For the full list of lenses that fit the a9, see the Sony a9 page.
Written by Claude from this site's pages.