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Around 2002, Olympus (and others) basically invented a new sensor format called 4/3, or Four Thirds. Read about it here.
This system used a sensor slightly smaller than the reigning enthusiast champ at the time, the APS-C sensor (such as used in cameras like the Canon Rebel series, or the Nikon D3000, D5100, etc).
It was meant to be an open or standardized system, but for the most part the only people that really invested into it were Panasonic and Olympus.
On a pure performance level, the 43 system was pretty darn good, especially the lenses. In fact, Olympus High Grade and Super High Grade lenses are some of the best lenses in the world, regardless of the format.
Unfortunately, this was a time where engineering expertise didn't listen to the marketing guys, and for once, they should have.
The 43 system had a smaller sensor than the APS-C and full framed cameras, but the cameras and lenses were just as large (and heavy!) as the competition - they used optical viewfinders and mirrors and bulky bodies, just like any other DSLR. Often, the performance of the 43 sensors just couldn't match the other systems when it came to noise, dynamic range, or focus tracking in fast situations.
In essence, the 43 system offered no real benefits from the 'other guys'. While, optically speaking, the 43 lenses were incredible performers, that wasn't enough to really sway lots of people away from their APS-C and FF systems or invite new people to the system.
Enter Micro Four-Thirds, or m43.
Thankfully, Olympus and Panasonic went back to the drawing board and came up with a different mount specification. They kept the 43 sensor size, but deemed this new mount m43.
M43 cameras don't use mirrors like traditional SLRs, allowing them a few benefits.
First, they're smaller, typically much smaller than a comparable DSLR. The optical viewfinders and mirror box assemblies in your typical DSLR take up room, which necessitates a larger camera body. Without the mirror system, one can drastically shrink the body.
Secondly, the new lenses for m43 are also a lot smaller than their APS-C or full-framed counterparts. Because these lenses are designed with portability in mind (and not just raw performance at any cost), and because they have to cover a smaller sensor than APS-C or full-frame, they are much smaller and lighter than a typical lens meant for APS-C or FF cameras. The difference can sometimes be staggering.
Of course, not everything is beneficial - there are always design tradeoffs for any camera system, and m43 is no exception.
First, there is the autofocus performance.
For the most part, AF performance is pretty good with m43. It uses contrast-detect autofocus (CDAF), which is basically the same type of autofocus used in P&S cameras. For the most part, though, m43 autofocus is more advanced than P&S cameras and can lock on extremely quickly.
When attempting to track subjects, though, CDAF really loses out to the other type of autofocus, phase-detect (PDAF). Read a fairly easy-to-digest article about PDAF over at Lensrentals.
There are pros and cons to each type of system. Typically, CDAF is extremely accurate, more so than PDAF. CDAF can, in many instances, match or even beat PDAF autofocus when it comes to static subjects in decent light.
In lower light, higher-end PDAF autofocus will beat most CDAF systems. When it comes to tracking, there's no real competition - PDAF is the only game in town.
Second, there are the (potential) issues of Depth of Field (DOF), Noise, and Dynamic Range.
Depth of Field - in a nutshell, the m43 format cannot attain super-thin DOF, at least in comparison to the full-frame format. It can, though, still easily blur backgrounds for pleasant photos. I've included a comparison further into my OM-D review to show this. Against APS-C, the m43 format loses a smaller level of DOF and is almost moot.
Noise and Dynamic Range - While DXO hasn't, as of 11 August 2012, release their sensor review for the OM-D, my feeling is it'll fall slightly behind in noise in relation to the best APS-C sensors, and certainly behind the best FF sensors.
This isn't to say, though, that the sensor is bad - noise performance is still very good, all things considering (especially since the sensor is at a theoretical disadvantage!) and dynamic range is very much improved with the OM-D, apparently due to the rumored use of a Sony sensor .
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