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Partial focusing points of AF sensor system
Full-surface focusing points of contrast AF system

Traditional SLRs use a dedicated AF sensor for autofocusing (phase-difference AF).
With this system, the number of distance-measuring points is limited by the size of the AF sensor - which becomes more complicated and bulkier as more distance-measuring AF points are required.
With the Micro Four Thirds system, on the other hand, there is no mirror and no separate AF sensor. Instead, autofocusing is performed in the image sensor itself. This means that distance measurement can be calculated using the entire sensor area, with no limit on the number of distance-measuring points. Not only is it more accurate and more efficient, it's faster too.

Other advantages of sensor-controlled AF include unrestricted setting of distance-measuring points (1) and grouping of specific areas (2), giving more flexibility and more precise control when setting the AF.

The distance-measuring speed of contrast AF has now evolved to the point where it is comparable with phase-difference AF, and, under certain conditions, is even faster. As the popularity of combined movie/still image recording grows, the advantages of the mirrorless system camera's contrast AF system will become of even greater importance, offering greater speed, durability, and simplicity than conventional phase-difference AF systems whose frequent mirror up-down operations constitute an insurmountable barrier.

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Technology evolving from face recognition to behavior prediction

One of the greatest advantages of the contrast-detecting AF system is face recognition.
Based on technology in the sensor that analyzes the shapes and contours of human eyes, mouths and faces, face recognition is currently used in many cameras to optimize face reproduction by detecting, tracking, and focusing on human faces, as well as by adjusting the exposure as needed, in some cases releasing the shutter only when smiling faces are detected.
In the future, as this technology continues to evolve, you can look forward to such exciting new features as an ultrafast AF system that predicts the subject's next action by analyzing the scene based on the status of the human skeleton. Without the sensor AF technology, none of this would be possible, and today many manufacturers and research organizations are pressing forward with development of new features derived from the potential of the contrast AF system.

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Viewfinder with stopped-down aperture in a conventional SLR
Magnified stopped-down view in contrast AF system

A prominent feature of an SLR is that the user sees the image reflected by the mirror in the mirror box. As it has been a premise to improve the view by opening the iris, checking the actual stopped-down condition for shooting requires mechanical stop-down using the preview button or some other method.
However, as the light volume of the image is lost due to the transmitting screen (frosted-type plane) on which the mirrored image is analogically formed and the optical viewfinder, checking is often hard even for experts when the aperture is stopped down by as many as three steps (see figure under (4)).

Since contrast AF is a completely digital system that uses image data on the sensor, the exposure simulation functions optimally even when the aperture is stopped down. This means the user can check the depth of field by viewing the image under the same optimum exposure used when the image is shot.
Other functions that make use of the direct applicability of the sensor information include Live View for checking the image on a larger screen and a magnification function. Together, these functions support more accurate focusing and shooting.

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The key to the many advantages of a mirrorless system camera, as shown on the left, are the elimination of a mirror box and AF sensor. In a traditional SLR, these would increase the camera size/weight and transmitting screen that would degrade the view.
Because a mirrorless system camera enables direct view of the actual image in the sensor, the image is not disrupted each time the shutter is released and can be viewed clearly at all times, including by magnification when the actual aperture value is applied.
The substantial reduction in the length of the flange back not only makes possible the unprecedented slimness of the camera, but also ensures that you can use (by means of a lens adapter) a tremendous variety of lenses, including classic lenses designed for past rangefinder cameras.

The shorter the flange back length, the better the performance. Not always. The length of the flange back must be optimized with respect to the diagonal length of the sensor; otherwise the design would be optically problematic and impact everything from design to production. Micro Four Thirds has been designed to take this into consideration, featuring a design that is compatible with a large variety of lenses, while maintaining high SLR performance.
* The use of certain classic lenses may impose restrictions on functionality. For details, please consult the lens manufacturer.

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