At night, the light behind the portrait becomes round? What is the approximate aperture?

What you should say is that the spots are round. ....

Mainly depends on your lens. There are many anti-glare lenses now. If you use that lens at night, you won't get any bright spots at all. The above picture shows that the effect aperture of Nicole 28 2 is around F8. Because he is an old man, it is easy to shoot a dazzling feeling at night. You can also try to install a piece of glass UV, which will be effective.