Danish-born Jacob Riis (1849–1914) was a pioneer in social documentary photography which included identifiable people and was one of the first photographers to use the new technology of magnesium flash. Riis photographed the flop houses where people were stacked at night
on every available horizontal space. Again these were usually immigrants and Riis showed the squalor they inhabited in his book How the Other Half Lives (1890) which featured the infamous Mulberry tenements in New York.
For a New York Times (2008) article on Riis visit:
Watch a 10-minute film clip about Riis and his use of the new magnesium flash: