I always thought that the reason people look so grim in antique photos is because it would have been exhausting to hold a smile for long exposures that I imagined were required by ye olde cameras. Nope! From the always-informative Smithsonian magazine:
…Exposures from the early days of commercial photography only lasted about 5 to 15 seconds. The real reason is that, in the mid-19th century, photography was so expensive and uncommon that people knew this photograph might be the only one they'd ever have made. Rather than flash a grin, they often opted to look thoughtful and serious, a carry-over from the more formal conventions of painted portraiture, explains Ann Shumard, senior curator of photographs at the National Portrait Gallery.
According to Shumard, it wasn't until Eastman-Kodak founder George Eastman's 1888 invention of the mass market portable camera that informal snapshots of smiling people became common.
image: Eugene Pelletan portrait c.1855 by Gaspard-Félix Tournachon