How is portrait photography formed?

In traditional painting, portrait painting has always been the main theme of painting performance, and portrait photography is a form formed under the influence of portrait painting. Photography shows a photographer taking a portrait of a gentleman in a studio in France. This photo, taken at 1843, records that at the beginning of the invention of photography technology, because of the long exposure time, the subject had to stay still for a long time in order to obtain a clearer effect, so the expression of the characters appeared dull.