Because the worst defects of ordinary lenses occur at the
edges, the 4/3 System standard pushes those edges farther
out, where they can do no harm. The 4/3 design requires that
the diameter of the lens mount be about 50mm, roughly double
the diagonal of the 33.87mm image circle. The more errant
light rays from the perimeter of the lens fall harmlessly out of
the image sensor’s territory. The light that does hit the sensor
comes more from the lens’s center where light rays come closer
to a path perpendicular to the sensor’s surface. 4/3 also
requires that the three-claw bayonet mount be standardized in
fit and distance from the focal plane so that a lens from one
brand of 4/3 System camera can be used with a different 4/3
System camera body.4
Because the 4/3 System lenses are designed specifically for digi-
tal cameras with sensors half the size of 35mm film, they have
the benefit of producing the same field of view found in a film
camera lens while using half the focal length. The 4/3 System
lenses can be made shorter and lighter. Many of the lenses have
their own microprocessor in addition to the three processors
found in the camera body. The lens sends the information about
itself through nine electrical contacts built in to the bayonet
mount. The camera’s processors and built-in software use that
information to correct lens distortions, such as pin cushioning.5
4/3 standardizes the diagonal size of the image sensor at 22.55mm so it fits within
the image circle that the standard dictates. The Olympus E-1 sensor has proportions
of 4:3, which makes it a little taller than and not as wide as the 35mm frame, cover-
ing about half the area. But 4:3 proportions are not required. A sensor can be any
shape, including square, as long as it fits inside the image circle. There is no require-
ment as to the technology used in the sensor—it can be CCD, CMOS, or Foveon X3,
all of which we’ll examine in Chapter 7, “How Light Becomes Data.”6
4/3 = What?
In the confusing tradition of aperture numbers, in which the bigger the number,
the smaller the aperture opening, the nomenclature for different types of digital
image sensors defies any comprehension. The 4/3 System standard is not a
measurement—1 1/3—of the sensor or anything else you’ll find on a 4/3
System camera. The same goes for other sensors, such as the 1.8, 2/3, and
1/2.5. Some experts claim the system refers to the aspect ratio of the sensors.
That might hold true for 4/3, but it breaks down with other sensors. It’s also sug-
gested that the designations come from a system used in the movie industry to
identify lenses and have no real meaning for the rest of us. It’s best not to give it
much thought. Just accept it and don’t ask questions.CHAPTER 8 HOW NEW TECH CHANGES PHOTOGRAPHY^123
OBJECT PINCUSHION DISTORTION