Pixel Quality
The
marketing race for "more megapixels" would like us to believe that
"more is better". Unfortunately, it's not that simple. The number of
pixels is only one of many factors affecting image quality and more
pixels is not always better.
The quality of a pixel value can be described in terms of
geometrical accuracy, color accuracy, dynamic range, noise, and
artifacts. The quality of a pixel value depends on the number of
photodetectors that were used to determine it, the quality of the lens
and sensor combination, the size of the photodiode(s), the quality of
the camera components, the level of sophistication of the in-camera
imaging processing software, the image file format used to store it,
etc.
Different sensor and camera designs make different compromises.
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Geometrical Accuracy
Geometrical or spatial accuracy is related to the number of pixel locations on the sensor and the ability of the lens to match the sensor resolution. The resolution topic explains how this is measured at this site. Interpolation will not improve geometrical accuracy as it cannot create what was not captured.
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Color Accuracy
Conventional sensors using a color filter array have only one photodiode per pixel location and will display some color inaccuracies around the edges because the
missing pixels in each color channel are estimated based on demosaicing
algorithms. Increasing the number of pixel locations on the sensor will
reduce the visibility of these artifacts. Foveon sensors
have three photodetectors per pixel location and create therefore a
higher color accuracy by eliminating the demosaicing artifacts.
Unfortunately their sensitivities are currently lower than conventional sensors and the technology is only available in a few cameras.
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Dynamic Range
The size of the pixel location and the fill factor determine the size of the photodiode and this has a big impact on the dynamic range.
Higher quality sensors are more accurate and will be able to output a larger dynamic range which can be preserved when storing the pixel values into a RAW image file. A variant of the Fujifilm Super CCD,
the Super CCD SR uses two photodiodes per pixel location with the
objective to increase the dynamic range. A more sensitive photodiode
measures the shadows, while a less sensitive photodiode measures the
highlights.
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NoiseThe pixel value consists of two components:(1) what you want to see (the actual measurement of the value in the scene) (2) what you do not want to see (noise).
The higher (1), and the lower (2), the better
the quality of the pixel. The quality of the sensor and the size of its
pixel locations have a great impact on noise and how it changes with
increasing sensitivity.
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Artifacts
Besides noise, there are many other types of artifacts that determine pixel quality.
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Conclusion
Unfortunately there is no single standard
objective quality number to compare image quality across different
types of sensors and cameras. For instance, a 3 megapixel Foveon type
sensor uses 9 million photodetectors in 3 million pixel locations. The
resulting quality is higher than a 3 megapixel but lower than a 9
megapixel conventional image and it also depends on the ISO level you
compare it at. Likewise, a 6 megapixel Fujifilm Super CCD image is based
on measurements in 3 million pixel locations. The quality is higher
than a 3 megapixel image but lower than a 6 megapixel image. A 6
megapixel digital compact image will be of lower quality than a 6
megapixel digital SLR image with larger pixels. To determine an
"equivalent" resolution is tricky at best.
End of the day, the most important thing is that you are happy with the quality level that comes out of your camera for the purpose that you need it for (e.g. website, viewing on computer, printing, enlargements, publishing, etc.). I strongly recommend that you look beyond megapixels when purchasing a digital camera. |
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