As the name suggests, a mirrorless camera is one that doesn’t require a reflex mirror, a key component of DSLR cameras. The mirror in a DSLR reflects the light up to the optical viewfinder. In a mirrorless camera, there is no optical viewfinder. Instead, the imaging sensor is exposed to light at all times. This gives you a digital preview of your image either on the rear LCD screen or an electronic viewfinder (EVF). Mirrorless cameras are called “mirrorless,” rather than DSLRs being called “mirrored,” simply because they came second.
Now, the term “mirrorless” is a little confusing. It didn’t really become used until the advent of mirrorless digital cameras with interchangeable lenses, but many styles of camera lack a reflexing mirror. Technically, a point-and-shoot is a mirrorless camera, as is a Leica rangefinder, and even older film models. However, the term “mirrorless” is generally used to describe digital interchangeable lens cameras (ILCs) that either has electronic viewfinders or simply no viewfinder, and that’s how we’ll be using it here.
If we already had DSLR cameras at all different price points, what was the point of removing the mirror and creating a new category of camera? The main reason is simply size. Without the beefy mirror box taking up space inside the camera body, mirrorless cameras can be made much smaller than their DSLR counterparts. Originally popular with beginners and casual photographers, this design has now grown on professionals, as well. We’ll get into additional advantages below.
Like DSLR cameras, mirrorless cameras use a bayonet-style mount for attaching different lenses. Each manufacturer has its own proprietary mount, so you can’t use just any mirrorless lens on any mirrorless camera. However, many third parties will produce lenses for those mounts, as well. The sole exception here is Micro Four Thirds, a mount that is shared by Panasonic, Olympus, and a few specialty players like drone maker DJI and cinema camera manufacturer Blackmagic Design. Any Micro Four Thirds lens can be used on any camera with the same mount, regardless of brand, with near full compatibility.
One of the neat things about mirrorless interchangeable-lens cameras is that the flange back distance (the distance between the lens mount and sensor) is much smaller than on a DSLR. This makes it possible to attach a wide variety of lenses from DSLR manufacturers using various adapters. Companies like Fotodiox and Metabones sell lens adapters for mirrorless cameras that will let you use everything from the latest Canon lenses to the oldest medium format lenses you can find. You often sacrifice some performance when you do this, but it can be a huge benefit for photographers with existing caches of lenses.
(Excerpted from DIGITAL TRENDS.)