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There is a common misconception that there is a big difference between the sizes of APS-C and Micro Four Thirds sensors. In fact, as shown on the left, the difference is even smaller than the difference between full-size and APS-C, and the Micro Four Thirds sensor is massive compared to popular compact camera sensors.
If the difference in area is as small as the difference with APS-C, components such as the lens and image processing engine have a much greater impact on image quality than the sensor.

The size of the Micro Four Thirds sensor was derived from the concept underlying the system's development, that is, to provide products that offer high image quality that anyone can use. and for fabricating a large diversity of lenses without compromising the image quality and productivity.

Today, the line between still images and movies has almost vanished, and the ability to offer superior movie characteristics (affinity) is becoming a critical factor.
As the problems of heat generation and battery consumption due to long-lasting exposure increase in proportion to the sensor size, it is now important to improve image quality by using a sensor as small as possible.
Many of the movie cameras designed for cinema and broadcasting were unable to solve these problems, so cameras using small 2/3" sensors have been used extensively.
Nevertheless, since the debut of Micro Four Thirds, which boasts a sensor that features compact size and low power consumption, while matching the image circle of PL-mount movie lenses, a great change was produced in the cinema and broadcasting industry. Now, as new Micro Four Thirds-compliant equipment is released in rapid succession, the Micro Four Thirds System is beginning to establish itself as the new standard of the movie and broadcasting industry in the digital era.
The movie and broadcasting industry has recognized Micro Four Thirds as the only standard capable of offering high image quality while solving the heat generation and battery consumption problems.

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The light passing through the lens is output from the output lens (rear-end lens) and forms a circular image on an imaging plane such as a sensor or film. The circular area containing an accurate image is referred to as the image circle.
In most cases, the sensor is sized so that it can deal with image deformation due to low light intensity outside the image circle. However, the area used in actual shooting is the area called the effective pixel area, which is inside the image circle. The size of this area is defined as the effective sensor size.

The diagram on the left shows the relationship between the image circle and effective sensor size. Due to the strict physical principles between the light passing through the lens and the subsequent output from the lens, it is generally necessary to design a lens with a large diameter and length in order to obtain a large image circle.
In addition, the flange back should also be optimized for avoiding unnatural refractions of light. Micro Four Thirds was developed with the goal to reduce lens size, while ensuring high image quality according to the physical principles above.

It is against physical principles to use a compact lens with high image quality with a large sensor. Even with many of the lenses for full-size cameras, problems such as insufficient peripheral brightness and imaging are treated by digital processing unless very expensive glass materials are used or the lens size is very big. This means, if you pursue high image quality, you need a lens that can meet the physical principles above with a size matching the image sensor size.
In fact, if this is ignored, your lens will cause noticeable imaging problems such as deformation of peripheral image.

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With film cameras, the light output from the lens is imaged by a chemical reaction. This process is sometimes arbitrary and does not pose any serious problems even if the light is incident obliquely on the film plane. With digital cameras, on the other hand, the output from the lens is imaged by an image sensor and oblique incidence of light on the sensor device affects the formed image due to reduction in the brightness of light or deviation of the optical axis.
To solve the above problem, the Four Thirds and Micro Four Thirds lens standards emphasize telecentricity (property of incident straight onto the image sensor surface) and the lens is designed so that it is capable of delivering a large amount of effective light (with a sufficient intensity and non-deviated optical axis) to the image sensor.
The diagram below shows the differences between a non-telecentric lens of the film era that outputs oblique light beams and a telecentric-type lens that always outputs perpendicular light to the sensor.

Telecentric-type lens

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Non-telecentric lens

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The non-telecentric lens cannot deliver effective light for the imaging size, but the telecentric-type lens can deliver effective light for the entire imaging range.
This is the secret of how Micro Four Thirds is able to produce sharp, high-resolution images using a compact image sensor. Even a non-telecentric lens can deliver light with little incidence angle with certain modifications such as increasing the lens size and setting the condensing lens attached on the image sensor side to a specific angle. However, in such cases, the lens and camera sizes would have to be increased considerably, at a level that is not at all practical in terms of either mobility or price.
Micro Four Thirds is able to offer both high image quality and high portability because its design integrates both lens and image sensor into a unified system where all elements work together to enhance overall performance.

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Full-sized SLR Maker A
APS-C mirrorless Maker B
Micro Four Thirds Maker C
* The photos above come from our own comparative experiments and the effect may differ according to conditions.

One way to express the performance of a lens is the MTF (Modulation Transfer Function) curve, which represents the degree to which a lens is capable of faithful reproduction of lines with a width of 1 mm (spatial frequency response). MTF is measured based on the contrasts of the lowest and highest frequencies emitted from the subject in order to estimate the resolution and clearness of the lens.
The diagram below is the MTF graph of the ZUIKO DIGITAL ED 150mm (equivalent to 300mm on a 35mm film camera), a highly rated high-performance compact super-telescopic lens.

The MTF graph shows that the resolved spatial frequencies (both the low- and high-ends) are high and that both high clarity and high resolution are balanced at a high level. If the characteristics on the low-end and high-end frequencies were different, the image quality would deteriorate in terms of clarity and resolution due to the imbalance between the contrast and resolution, while balancing of both characteristics at a high level is indispensable for high-quality lenses.

Sharpness and resolution are affected by higher frequencies. They can be identified easily by duplicate photography even without referring to the MTF characteristics.
The photos on the left show a comparison of the resolution performance of full-size, APS-C and Micro Four Thirds cameras using the actual (duplicate) pictures shot under the same conditions (same focal depth when converted to the 35mm film camera and same depth of field setting).
The subject used for this test was an A2-size (420 mm x 594 mm) map. The shooting distance was 42 cm, the focal depth was 24mm when converted into the 35mm film camera, and the depth of field was unified to the case of F4).

When we observe the magnified photos, we see that the full-sized SLR and Micro Four Thirds manifest almost equivalent resolution without any deformation of the peripheral image. However, the peripheral area captured with the APS-C mirrorless camera is not imaged precisely and looks blurry overall. As seen here, excessive reduction of the mount size and/or flange back can affect the optimum balance with respect to the sensor size, tending to degrade the image quality in the peripheral areas.

The example here clearly demonstrates that lens performance is indispensable to image quality improvement regardless of the sensor size. However large and high-performance the sensor is, pictures with high image quality cannot be obtained if the lens performance is compromised by excessive size reduction. On the other hand, Four Thirds and Micro Four Thirds make it possible to design high image quality-capable lenses thanks to the optimum balance between the mount/flange back size and the sensor size.

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As discussed in (2), the flange back is an important factor for high image quality and related to many other matters such as the sensor size and lens design. Theoretically, a short flange back or back focus can facilitate improvement of the wide-angle lens performance by rendering the front and rear of the lens symmetrical. However, if it is extremely short compared to the diagonal length of the sensor, unfavorable effects such as distortion and deformation of peripheral image will be noticeable.
With Micro Four Thirds, the balance between the sensor's diagonal length and the flange back/back focus lengths are defined in order to enable improvement in the freedom of design/fabrication and the performance of lenses developed in the future.
The short flange back allows Micro Four Thirds to mount classic lenses, providing a mount system that allows users to safely and easily enjoy the use of a large variety of lenses.
* The use of certain classic lenses may impose functional restrictions. For details, please consult the lens manufacturers.

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Although reduction of the flange back should theoretically enable a design that supports wide-angle shooting, the fact is that the freedom obtained by reducing the flange back cannot be utilized properly unless the design is very advanced and expensive glass materials are used extensively. This makes such a design problematic in terms of both market price and the number of units that can be produced. In designing the Micro Four Thirds System, these considerations were carefully weighed, and a balance struck that ensures both high quality and affordability.

The photos below show a comparison of the effects of a wide-angle single-focus lens in Micro Four Thirds (left) and APS-C (right).
Although the overall sense of distortion feelings of overall images in each image is different due to the slight difference in focal length, the difference in resolution is clarified by enlarging the peripheral areas of the photos of the bridge in the upper row. While a remarkable drop in the peripheral brightness and deformation of the peripheral image can be seen in the APS-C photo on the right, the resolution of the peripheral area in the Micro Four Thirds image on the left is just as good as that of the central area.
In addition, the photos of TV screens in the lower row show that the distortion is unable to be corrected and remains as a curved image.
An extreme flange back setting not only make lens design/production difficult and thereby degrades image quality, it also makes it impossible to exploit the performance of the sensor properly.

Micro Four Thirds 12mm (24mm : 35mm equivalent)
APS-C mirrorless 16mm (24mm : 35mm equivalent)
* The photos above come from our own comparative experiments and the effect may differ according to conditions.

Micro Four Thirds 12mm (24mm : 35mm equivalent)
APS-C mirrorless 16mm (24mm : 35mm equivalent)
* The photos above come from our own comparative experiments and the effect may differ according to conditions.

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As already discussed in (1), the difference in sensor size between APS-C and Micro Four Thirds is fairly small, with an area ratio of only about 160%. In terms of the light intensity received by the sensors according to their area ratio, this difference corresponds to less than one EV step. Similarly, the difference in the amount of background amount is also equivalent to less than one step.
Unlike full-size cameras, focusing is not difficult with either Micro Four Thirds or APS-C thanks to their small depths of field. This gives both standards an advantage in terms of AF speeds and both can be regarded as easy-to-use standards with few shooting failures.

The photos below are actual shots of the same scene using APS-C and Micro Four Thirds. The color tones are different because of the differences in the lens characteristics and the image engine in use, the difference in the depth of field is about one step in both shooting conditions.

These photos mean that, if it is required to shoot a photo with more background defocusing, all that is necessary is to open the iris and that, on the contrary, with Micro Four Thirds is easy to shoot photos without defocusing by making use of the one-step deeper field of depth.

APS-C mirrorless: 72mm (35mm equivalent), F5.6.
Micro Four Thirds: 72mm (35mm equivalent), F5.6.
Micro Four Thirds: 90mm (35mm equivalent), F1.8.
* The photos above come from our own comparative experiments and the effect may differ according to conditions.

42mm (35mm equivalent), F8
42mm (35mm equivalent), F4.5
40mm (35mm equivalent), F5.6
58mm (35mm equivalent), F5.6
* The photos above come from our own comparative experiments and the effect may differ according to conditions.

In fact, however, to increase the defocusing amount in actual shooting, it is more also effective to turn the zoom ring toward the telephoto side or use a telephoto lens regardless of the camera is full-sized or Micro Four Thirds.
The photos of flowers on the left show comparisons by aligning the subject image size using a standard zoom lens covering about 3X focal lengths (14-42mm).
Background focusing is possible by simply opening the iris (comparison of upper-row photos), but increase of the focal length is effective if more impressive defocusing is required. Even when the same F-value is applied, the background defocusing image can vary greatly with a difference in focal length of only 18mm (comparison of lower-row photos).
A large number of compact, high-performance telephoto zoom lenses have been released for Micro Four Thirds, and many of these adopt the circular iris mechanism capable of producing beautiful defocusing effects.
Instead of being preoccupied with the iris setting, users can concentrate on effects of focal length and try varying the shooting positions. These are the first steps toward creating your own masterpieces, and are the techniques practiced by professional photographers.

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One of the greatest achievements of Four Thirds and Micro Four Thirds is the reduction made possible in the size of high-image-quality lenses. Four Thirds achieved superior SLR quality with a telescopic lens with about half the volume of previous lenses, and Micro Four Thirds improved the usability and size/weight reduction of lenses from the wide-angle to standard range while also optimizing them for movie recording. Both of these successes are based on our ongoing commitment to "high image quality portable by people".

Given a concrete shape by the adoption of mirrorless design, the demand for "a simple way to enjoy high image quality from a more compact camera" has led to the market release of cameras incorporating ever smaller sensors based on new technology, reflecting various benefits of technological advances and a growing demand for movie recording.
This supports the claim of the Four Thirds/Micro Four Thirds camp that "it is not only the sensor size that determines the image quality".

Nevertheless, it is also a matter of fact that the physical principles of electronics and optics continue to apply unless there are revolutionary inventions or discoveries related to the fabrication of sensors and lenses.
Micro Four Thirds was developed to satisfy the two goals of "image quality worthy of appreciation" and "trouble-free portability" at a high level based on the philosophy of Oskar Barnack. In this context, we believe that it is the best system unless the current scientific background changes dramatically.

Even if you purchase an SLR expecting the fun of using interchangeable lenses, it is meaningless if you feel "the lens is too heavy/bulky for carrying". Micro Four Thirds is a standard based on the idea of implementing the high image quality of an SLR in the smallest size practical. For instance, when you want to carry your most frequently used wide-angle zoom lens (18-35mm), a standard zoom lens (28-85mm) and a telephoto zoom lens (150-600mm), the total weight is only 850 grams. Furthermore, the total volume is as small as 7.2% of full-size standard lenses and 42% of the APS-C standard lenses. These lenses are so small and light, you'll have no trouble carrying them along when hiking a mountain trail.

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A system of innovative high image quality and compact size is achieved thanks to comprehensive size reduction of all components including lenses, Micro Four Thirds can freely use any of the wide selection of dedicated lenses, as well as top-quality Four Thirds lenses. A special adapter even makes it possible to use AF with these lenses.

Another advantage of Micro Four Thirds is the availability of many classic lenses, as well as a large number of accessories.
* The use of certain classical lenses and accessories may impose restrictions related to the functions. For details, please consult the lens manufacturers.

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