Canon PIXMA MX870 - Manual (Page 378)

It is recommended to select Normal for most cases. Select Strong if you have selected Normal and
the noise still bothers you.
Depending on application software or resolution of image data, effects of digital camera noise
reduction may not be obvious.
When this function is used for other than photos taken by digital cameras, image may be distorted.
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Page 378 of 1127 pages Reducing Photo Noise