In your collections, you will often come across photographs in a format that you cannot identify. These are primarily early photographs. By understanding the type of photograph you have, you can begin to learn when or why it was taken. Knowing the format is also helpful in finding out the best way to preserve the images.
Photographs contain a wealth of information, but you need to build your visual literacy to extract clues. The social history context that photographs provide is especially important for groups who have historically not been represented in text-based archival collections, such as women or people of color. I've gathered some of the techniques I use when conducting historical analysis of photographs.