Micro Four Thirds was designed specifically to remove the need for a mirror box and pentaprism viewfinder. By eliminating the mirror box, they could create a much smaller, more shallow body. Early interviews with the Olympus CEO (?) talked about a small interchangeable lens camera that you could put in a jacket pocket and carry to dinner. This was at a time when DSLR's ruled, and I carried a 30D in my shoulder bag everywhere. Micro Four Thirds system cameras were the first mirrorless cameras on the market, if you don't count the Leica M8.
Exactly ... although no one considered the Leica M8 to be a "mirrorless" camera when it came out. It was simply the first Leica
rangefinder camera (following the Epson R-D1 a couple of years prior) with a digital sensor instead of film as the recording medium, at the time. The term "mirrorless" came into being after the Panasonic G1 and Olympus E-P1 models (and then follow-ons) circa 2008-2009, and indicated a camera with a Live View imager—used for focusing, framing, and metering as well as recording—as the defining factor ... neither the Leica M8 nor M9 supported Live View.
Leica Ms were included as Live View cameras with the introduction of the M typ 240 (introduced about 2012, IIRC) and later models, although they remain rangefinder cameras in their native operation even today with the M11 range. The first
true Live View Leica was the SL typ 601 in 2015, followed by the TL and CL models.
The advantage of mirrorless over DSLR designs (aside from the potential compactness) is that fact that, just as for rangefinders, it allows more degrees of freedom in lens design compared to having to accommodate the swinging mirror's mechanical clearance requirements. This is why rangefinder lenses (particularly short focal lengths) have been subtly superior (on average) to comparable short focal length (D)SLR lenses, and the same goes for lenses designed for mirrorless bodies.
Meditating on this, I've lived through all of this technological history and owned/used/made a living from many of these groundbreaking new cameras along the way. The years since 1998 to the present, 27 years or so, represent 10x or greater rate of change in camera technology compared to the previous 36-40 years of camera technology in my lifetime. It's just amazing ... The Nikon/Leica kit that once remained nearly unchanged (and still current!) for 30+ years became an ongoing carousel of new models, new features, new capabilities on an annual or even shorter baseline. It's nothing short of amazing. 🤯
In my old age, I'm enjoying the 'simplicity' and lack of hectic change of pace provided by elderly cameras from the '40s, '50s, and '60s. It really was a period of intense development and change, growing complexity, but it seems so quiet and staid compared to what happened in the last quarter century. 😇
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