Long winded explanation of "Four Thirds" and other image sensor sizes (if you just want to skip to the actual "Four Thirds" explanation jump down to the ***):
35mm film has a frame size of 36mm wide x 24 mm high (a 4:3 ratio). This results in an overall picture area of around 864 square mm. Most (all?) digital cameras use sensors with form factors in this same 4:3 ratio.
A full frame DSLR has a sensor about the same size as a 35mm film frame, 36mm x 24 mm, so with a standard lens of any particular focal length, you'd get essentially the same image size as you would with a 35mm film camera. Many digital cameras, though have smaller sensors than this. If you mounted that same lens from a 35mm film camera on a digital camera with a smaller than 36x24mm sensor, only the central part of the image that falls on the sensor would be captured and the stuff around the outside would be lost. This means that on such a camera you can have smaller diameter lenses that are less bulky and less expensive that only project their images on the actual sensor area and still get the same overall image size. But that compactness comes at a price. In general, with the smaller sensor area, the fewer pixels (individual picture elements or "dots") it can capture. This is important if you are blowing your picture up to make very large prints, or will be cropping it down to very small portions of the overall original image, but for most of us who now show our pictures on computer monitors or tv screens, or just upload to social media, this extra detail is lost anyway and only serves to unnecessarily send our image file sizes into the stratosphere.
For instance I have a "point-and-shoot" camera (Panasonic Lumix ZS40, a couple of generations old but still plugging away very well). Its sensor is only 6.17 mm x 4.56mm for an area of about 28 sq mm, much smaller than a 35mm film frame or a full sized DSLR. With the sensor in that particular camera, that still yields a 4:3 image size of 4896 x 3672 pixels, plenty large to get a full HDTV size of 1920x1080, or even a 4K TV image of 3840x2160, but you probably wouldn't want to print this out to much over an 16" or so print. (There are other factors besides number of pixels that affect image quality but that's beyond the scope of this post.)
So how is that 6.17mm x 4.56mm sensor in my ZS40 described? They say it is 1/2.3" -- say what??
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Turns out this terminology is a carry over from early video days when TV camera sensors were described by the outside diameter of the vacuum tube sensors used at the time. The 1/2.3-inch sensor in my camera is supposed to be about the size that an old video sensor would have if its overall tube diameter was about 0.435" (1/2.3 inch = about 0.435") Other common ones are 1/1.7" (a bit larger than mine), and 1/1.25" (a bit smaller than mine). Many mobile phone cameras have even smaller sensors. This nomenclature makes little sense today but that's the way it's done.
Okay, finally we get to "Four Thirds". There are digital cameras with sensors considerably larger than my Lumix ZS40 but smaller than a "full size" DSLR. Such cameras have sensors listed as the equivalent in size to those of "1 Inch" vacuum tubes (116 sq mm) and "Four Thirds" (i.e 4/3" = 1.33") vacuum tubes (225 sq mm), etc. There's also an "APS-C" format that breaks from this nomenclature and is larger yet. These provide improved image resolution over the point-and-shoot cameras with smaller sensors, but also are larger and bulkier, with larger and bulkier lenses, and higher (often MUCH higher) prices.
Many pundits suggest for "serious photography" the minimum sensor size is the "1 inch type". A "Four Thirds" camera has a sensor area almost twice that of a "1 inch" (225 vs 116 sq m).
Here's a fairly decent reference on all this:
http://photoseek.com/2013/compare-d...ame-35mm-aps-c-micro-four-thirds-1-inch-type/
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) for $149. Killer deal. Normal price is $200. I picked it up to try on this trip in a few weeks, so I'll let you know when I get back how it worked. Tests around my house looked great. I haven't tried it underwater yet. 
