The Micro Four Thirds system (MFT) is a photography standard created by Olympus and Panasonic for mirrorless, interchangeable lens, digital cameras and camcorders. Micro Four Thirds does not require space for a mirror and a pentaprism, which allows smaller camera bodies to be designed.
Compared to existing compact cameras, which have smaller image sensors and fixed (or non-interchangeable) lenses, Micro Four Thirds cameras are intended to offer a compact solution with interchangeable lenses and a larger sensor more similar to a DSLR camera. A Micro Four Thirds camera’s sensor is around nine times larger than in a typical point-and-shoot digital compact camera, giving vastly improved image quality and far greater ISO sensitivity, comparable to an entry level DSLR system.
A Micro Four Thirds camera provides near DSLR-quality pictures from a significantly smaller camera, however they are still larger and heavier than current compact cameras.
The Four Thirds name stems from two ideas: the size of a camera’s image sensor and its aspect ratio. Four Thirds casts off the 3:2 aspect ratio still used by consumer-level digital SLRs in favor of a more enlargement-friendly 4:3 ratio. High-end broadcast TV cameras use an image sensor with a diagonal measurement of 2/3 of an inch. The Four Thirds systems use an image sensor that is twice the size of a standard 2/3-inch TV camera sensor. Nothing about the Four Thirds image sensor actually measures 1 and 1/3 inches, but the idea that two 2/3-inch sensors put together equals a 4/3 sensor fits nicely with the 4:3 aspect ratio.
Advantages of Micro Four Thirds over DSLR cameras:
- Smaller and lighter cameras and lenses;
- Shorter flange-focal distance means that practically all manual lenses can be adapted for use;
- Shorter flange-focal distance allows for cheaper, smaller and lighter normal and wide lenses;
- Smaller sensor size allows for cheaper, smaller and lighter telephoto lenses;
- Absence of mirror eliminates “mirror slap” noise and vibration;
- Electronic viewfinder can provide real-time preview of exposure, white balance and tone;
- Brighter electronic viewfinder in low light.
Disadvantages of Micro Four Thirds compared to DSLRs
- The sensor may be smaller than APS-C sized sensors, this can lead to a potentially lower image quality than APS-C based DSLR cameras with a similar pixel count;
- Contrast detect autofocus is generally slower (albeit more accurate) than the phase detect systems used in most DSLRs.
- Due to the absence of a mirror and prism mechanism, there is no ability to use a through-the-lens optical viewfinder. The electronic viewfinder or a separate optical viewfinder must be used instead;
- Changing lenses can expose the sensor to dust (though equally true of all ‘mirrorless’ interchangeable lens digital camera designs today), compared to DSLRs which have both a mirror and a closed shutter protecting the sensor (all current Micro Four Thirds cameras include a  dust reduction system);
- Larger crop factor (2x multiplier) means deeper depth of field for the same equivalent field of view and f/stop;
Advantages of Micro Four Thirds over compact digital cameras
- Greatly increased sensor size (5–9 times larger) allowing improved low light performance and greater dynamic range;
Shallower depth of field possible (e.g. for portraits).
Disadvantages of Micro Four Thirds compared to compact digital cameras
- Physical size (camera and lenses are both larger due to increased sensor size);
- Extreme zoom lenses available on compacts (such as currently available 30× models) are not available on large sensor cameras due to physical size, cost, and practicality considerations.
Implications for Underwater Photography