Single-lens reflex (SLR) cameras were first introduced in the late 19th century, but they did not become popular until the mid-20th century with the introduction of 35mm film. SLR cameras use a mirror and prism system to reflect the image from the lens up to the viewfinder, allowing the photographer to see exactly what the lens sees.
In the 1950s and 1960s, SLR cameras became more sophisticated with the introduction of features such as built-in light meters and automatic exposure modes. The popularity of SLR cameras grew rapidly, and they became the go-to cameras for professional photographers.
In the 1980s and 1990s, SLR cameras became even more advanced with the introduction of autofocus technology, motorized film advance, and electronic shutters. These features made SLR cameras easier to use and more versatile than ever before.