There's a reason people in old photos look so serious, and it's not because having a sense of humor is a modern convention.
For much of the earlier half of the 1800s, cameras were too slow and clunky to capture "fleeting expressions like a genuine smile," TIME reported.
It was considered perfectly acceptable to keep your face neutral in photos back in the day because otherwise, you'd risk having them come out blurry. There's also the fact that, in elite circles, big grins were considered somewhat lower class.
So whether the subject was trying to affect an upper class look or simply opting for clearer photos, smiling was often a no-go in formal pictures throughout the 1800s. TIME reported the solemn look ultimately began to fade when improvements in photographic technology allowed more people to purchase their higher quality cameras in the 1900s.
Source : http://www.businessinsider.com/old-fashioned-manners-people-dont-use-anymore-2017-10