Every wonder what that second mirror is in your DLSR? It is the mirror for the auto-focus sensor. DSLRs have a separate sensor that resides at the bottom of the mirror box, whereas mirrorless cameras have theses AF-sensors built into the images sensor. This two-mirror setup has evolved over the years from its roots as a manual focus aid.
There is nothing like looking through a big, beautiful, ground glass focusing screen of an old film camera – there is something about the allure of micro-prisms and split focus that is lost on modern DSLR and mirrorless cameras. Most photographers now live and breathe by the auto-focus speed of cameras and lenses, but it wasn’t always this way.
The first auto-focus lenses were notoriously heavy and slow; many ‘professionals’ would scoff at it as inferior, much like how auto-focus is regarded in video production. Early film cameras, like the Canon T80, had to rely on awkward setups with an external motor built into the lens, often requiring its own power source. As the years went, companies like Canon improved their systems speed and accuracy to the point of professional use. In the little animated short below, Canon USA illustrates how auto-focus has changed over the last 30 years.
Canon demonstrates the evolution of early AF systems to their new Dual Pixel array found on their 5D Mark IV. Various AF features are briefly explained, such as using focus tracking AF-points.
Today is easy to take fast and precise auto-focus for granted, but the majority of photographers can appreciate how it has made our lives easier so we can focus on other matters a composition and storytelling.