An article from Open Culture describes why “A Rare Smile Captured in a 19th Century Photograph” is peculiar in the history of photography:
For one thing, we are not used to seeing them in old photographs, especially ones from the 19th century. When photography was first invented, exposures could take 45 minutes. Having a portrait taken meant sitting stock still for a very long time, so smiling was right out. It was only near the end of the 19th century that shutter speeds improved, as did emulsions, meaning that spontaneous moments could be captured. Still, smiling was not part of many cultures. It could be seen as unseemly or undignified, and many people rarely sat for photos anyway. Photographs were seen by many people as a “passage to immortality” and seriousness was seen as less ephemeral.