Specifications

The new standard for image quality and detail
A sensor that enables
true image quality
The image sensor is the most
important part of a digital SLR
camera. What, then, should
a photographer choosing a new
camera look for in an image sensor?
Most camera makers today
emphasize megapixels, which are of
course a factor in determining image
resolution. Sigma believes, however,
that a theory of image quality
that begins and ends with the
megapixels is grossly incomplete,
since it is the structure of the image
sensor that has the greatest impact
on finished image quality.
From the first generation onward,
Sigma digital cameras have featured
the Foveon X3® direct image
sensor. This sensor leverages the
special qualities of silicon, which
is penetrated to different depths
by different wavelengths of light.
It was the world's first and remains
the world's only image sensor to
capture fully the three basic colors
of light — red, green, and blue — in
each pixel location without relying
on color filters.
In fact, the Foveon X3® direct image
sensor receives red, green, and blue
wavelengths of light vertically — just
like modern color film. Considering
it far superior to the much more
common Bayer filter sensors
available, most experts agree that
the Foveon X3® direct image sensor
has a unique structure that makes it
the image sensor of the day — and
of the future.
Other digital cameras'
monochrome image sensors
With the exception of Sigma's
products, almost all digital
cameras available today contain
monochrome image sensors.
Unable to distinguish between
colors, these sensors receive light
through a filter that has an even
distribution of red, green, and blue
filtering units. As a result,
the sensor records color not
vertically, but horizontally.
Since each light-sensing photodiode
has above it a tiny filter unit that
lets in only one color, each pixel
captures only one color, and data for
the other two colors of light is never
collected at all. A color interpolation
process known as demosaicing
guesses at the missing colors based
on the colors of neighboring pixels,
adding them back in.
Developed and refined over an
extended period, this method of
processing images has matured,
and the interpolation of colors
is now fairly accurate. Inevitably,
however, interpolating colors based
on neighboring pixels results in the
loss of subtle color details found in
the original subject.
Demosaicing compromises
color detail.
Due to the demosaicing process,
conventional digital cameras using
color filter arrays also generate
color artefacts — colors not found
in the original subject at all.
The problem arises when areas
of the subject have a higher
frequency of color detail than can
be adequately covered by the grid
of red, green, and blue units
in the Bayer filter.
A conventional Bayer filter digital
camera has an additional optical
low-pass filter between the lens
and the sensor. This filter acts on
the high-resolution image from
the imaging lens, eliminating high-
frequency, detailed elements likely
to generate color artefacts before
they can reach the image sensor.
Although it effectively suppresses
color artefacts, the optical low-pass
filter also systematically reduces the
resolution of the image.
The Foveon X3®
direct image sensor generates
truly emotional images.
Sigma digital SLR cameras produce
truly emotional images, thanks
to the Foveon X3® direct image
sensor. Completely different in
structure from conventional image
sensors, the Foveon X3® direct
image sensor offers truly distinctive
image quality characterized by fine
detail—detail essential to capturing
the emotional qualities of the
subject. Sigma digital SLR cameras
are designed to reproduce what the
photographer shoots, right down
to the feeling in the air—a result
that is only possible with a vertical
color-capture system that does not
require color interpolation, and an
image-processing system that does
not require an optical low-pass filter.
In contrast, a conventional image
sensor performs guesswork on
colors and even cuts out high-
frequency areas completely.
To a degree, colors can be adjusted
in post-processing. But the subject's
details and nuances, once lost, can
never be recovered. To compensate,
conventional cameras utilize
sharpness processing to give the
appearance of a higher resolution.
As a result, these cameras produce
images that have artificially sharp
edges and give an uncanny and
unnatural impression.
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