In old photographs of people that I’ve seen the subjects rarely if ever smile. It seems that smiling while having one’s photo taken didn’t become common until sometime during the early 20th century.
Any speculation as to why smiling was so uncommon in early photos? Maybe exposure times were so long that smiling became uncomfortable or perhaps it was just unfashionable.
Old time photos requirted long exposure time - sitting or standing for an hour to get one’s photograph taken was not uncommon. Smiles are hard to maintain for that amoput of time, so most people didn’t smile in photos. That is what I learned fom my art class, anyway.
That, and I think it was probably also a more serious occasion, being more rarely done than in today’s snapshot society.
There were a lot of reasons.
Smiling seemed sort of artificial. People wanted realism in photos. A person doesn’t usually stand around and smile all the time, so having a photograph of a smiling subject would seem a bit odd.
Photographs were seen sort of as the new version of paintings. Folks who could never dream of affording a painting might be able to afford one of these newfangled portraits. They took their cues from the sitters in paintings-- solemn, still, stiffly posed, dressed in their best clothes.
Secondly, dentistry was pretty much limited to pulling teeth when they became to painful to endure. Had those people been smiling, showing their teeth, you’d probably be appalled at how they look.
Scarlett67 is right: having one’s picture taken was serious business. Many people only had one photo taken in their entire lives. (Which is why photographs of the dead were so popular-- it might be the only photograph of that person they had.) Some subjects, especially children, were probably nervous or a little frightened by this new process. They probably didn’t feel like smiling to begin with.
It does depend, when one talks about how long it took to sit for a photo, what one means by “old time photos”. Daguerrotypes took a long time to sit for, and so did other very old photograph types, but by the time we got to tintypes and such, photographs were much cheaper and, while the exposure time was much longer than we’re used to, wasn’t soooo long either - IIRC, 10 seconds or so.