In some of the earliest photographs from the late 19th/early 20th centuries (and in some daguerreotypes earlier than that), most people are pictured with straight, stoic faces. These days, however, it's normal to smile for all photographs, even formal ones for informational purposes.
How did this begin? When did this start? Or has it always been this way, and we're merely overlooking it?
People will tell you how developing the photo took a long time, this is actually not true since the exposure time were shortened to a few seconds by 1850's. The biggest reason is photography at the time was an alternative to getting your portrait painted. They viewed it as a serious thing thing to do, so they kept the same face they would as if they were having their portrait painted. People didn't smile because they didn't know they were suppose to smile.
hi! there's always room for more info, but do check out previous discussions in the FAQ (see link on sidebar)