My machine is Canon 5D2 and the lens is 17 40 F4. Is it suitable for taking portraits at night? How?

17-40 is not suitable for portraits, but the scenery is good. Flash is essential for taking portraits at night. Blurred background, using 50mmf 1.4 or 85mmf 1.8, the latter is better, and portrait definitely has many advantages over 17-40. The flash can be matched with 430, which is cheaper and 600 is better. It is recommended to use M file when portrait, and the aperture of the two fixed-focus lenses can be f2.8 and iso. If you consider the scenery and portraits, 24-70f2.8 is very comfortable.