Whenever you see early-era photographs the individuals in the picture generally aren’t smiling, such as in the case of President John Quincy Adams – the first American President to be photographed – above. Michael Zhang answered why this was:

• It would take up to fifteen minutes for the photograph to be taken. In that time if the subject moved around then the picture of them would be blurry. Since it’s difficult to keep a smile in place for 15 minutes, photographers advised people to keep a straight face.
• When people first started having their photographs taken modern dental practices hadn’t become widespread. Instead of getting root canals and caps people would just get their teeth pulled. Rather than spoil the picture with missing teeth, people preferred to keep their mouth closed.
• Getting a photo taken used to be fairly expensive, and thus it was a rare, formal occasion where people felt it would be inappropriate to smile.

Read more about when cameras first became widespread, and the time when people used to say “prunes” instead of “cheese” over here.

Source: Peta Pixel