Tempesta, I'm sure you're correct in your assumption that there's more to the "serious" expressions on people's faces in Victorian photographs than the exposure time of contemporary cameras. Another factor might be that having your photo taken back then was almost infinitely less common than now. In Victorian times it was a big occasion, was very often related to an important event, and took time and money. There'd be no thought of clowning around.
That is exactly what I had always suspected, Corio, a sober, more serious expression tends to lean toward this. It actually brings to mind the Annie Leibovitz exhibition of political figures in America, she has taken many such portraits, as the official Whitehouse photographer.
Zxath, thank you for your insight. Not being a photographer, exposure times had not even crossed my mind.
Tempesta, you're quite right, when did smiling become the norm? I can't help but find many Victorian photos have quite stern expression, though.