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Moire
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If a scene contains
areas with repetitive detail which exceeds the resolution of the
camera (1),
a wavy moiré pattern (2) can
appear, as shown in crop A. There is no moiré in crop B of an image
of the same scene taken with a camera with a higher resolution.
Anti-alias (3) filters reduce or
eliminate moiré but also reduce image sharpness.
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A. Example of moiré waves. |
B. No moiré in
this crop taken with a higher resolution camera. |
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| Maze Artifacts
Sometimes, moiré can cause the camera's internal
image processing to generate "maze" artifacts. |
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| Example of maze
artifacts |
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| Technical footnotes for
advanced users: |
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(1) When projected onto the sensor. |
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(2)
In technical terms this means that the spatial frequency of the subject
is higher than the resolution of the camera which we defined by the
Nyquist frequency. This high frequency detail causes lower harmonics to
appear (frequency aliasing) in the form of moiré waves. |
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(3) They are named
anti-alias filters because they reduce "frequency aliasing"
mentioned in the above footnote. Because anti-alias filters
tend to soften images, they incidentally have an indirect
"image anti-aliasing" effect, but that is not the reason
they are named this way.
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