Since William Dickson and Thomas Edison introduced 35mm film in 1892, this gauge (equal to 1-3/8 inches) has become the standard for still and motion photography. The result? In most cases, a 20th-century photo’s size and format won’t help you pinpoint a date for the image.
Fortunately, changes in fashion and home décor have accelerated, making it relatively easy to date a photo based on clothing and background items displayed. (See our article on dating 20th-century photos.) You also can ask relatives—who may remember the whos and whats for these relatively recent photos—compare the estimated ages of people shown to information on your family tree charts to figure out when the picture was taken.
But other film sizes and types of cameras did stick around, such as Kodak’s 126mm film introduced in 1963, 110mm film (1972), and the disc films of the 1980s. You may be able to date prints or negatives from non-standard film based on size. Look up the print size on this handy chart on Wikipedia, then see the type (or types) of film that produced prints this size, and the dates in use. Because Wikipedia has this pegged as a page that needs citations, treat the date as a hypothesis and confirm it using visual details in the image. The image area (not including the white border) for Polaroid’s 600 series of prints is 3.1×3.1 inches; for 669 film, it measures 3¼x4¼ inches.
From the March 2010 Family Tree Magazine