If a scene contains areas with repetitive detail which exceeds the resolution of the camera, a wavy moiré pattern 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 filters reduce or eliminate moiré but also reduce image sharpness.
|A. Example of moiré waves.||B. No moiré in this crop taken with a higher resolution camera.|
Sometimes, moiré can cause the camera's internal image processing to generate "maze" artifacts.
|Example of maze artifacts|
Technical Footnotes for Advanced Users
- (1) When projected onto the sensor.
- (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.
- (3) They are named anti-alias filters because they reduce "frequency aliasing" mentioned in the above footnote. Anti-alias filters tend to soften images and create an "image anti-aliasing" effect.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com