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View Full Version : What is moir้?



RUTch
4th April 2004, 03:37
Question
In some of my photos I have an odd red/blue stripe or pattern. What is this?

Answer
Sometimes odd stripes or colors will appear in a digital image, either from a high-end digital camera, or from a scanned image. This effect is called moiré and is caused when a fine pattern in the subject (such as the weave in a fabric or very close, parallel lines in architecture) matches the pattern of the imaging chip. When two patterns meet, often a third, new pattern is created. This third pattern is called moiré. Notice in the illustration below the round, circular patterns (on the right) created as the two grids are combined; this is moiré.

http://www.nikonusa.com/kdb/moire/moire_forms.gif

In order to reduce (or eliminate) moiré, a special anti-aliasing filter is mounted in the camera. If too strong a filter is mounted an overall soft image will be produced, but with no moiré. If a weaker filter is chosen the image will be sharper, but there is more of a chance for moiré to happen in some circumstances. Nikon has chosen to produce the sharpest image that can be made, even though there may be some moiré in parts of some images.

http://www.nikonusa.com/kdb/moire/moire_sample.jpg

To help reduce moiré there are many techniques to use:

Change angle of camera.
Since the angle of the camera and subject causes moiré, slightly changing the angle of the camera (by rotating the camera) can remove or change any moire that is present.
Change camera position
Again, changing the angle relationship by moving left or right, up or down can reduce moiré.
Change focus point
Moiré is caused by very sharp focus and high detail on fine patterns; slightly changing the focus point changes the sharpness and can help to remove moiré.
Change lens focal length
Different lenses or focal length settings can be used to alter or remove moiré
Remove with software
Nikon Capture (as well as several third-party, Adobe® Photoshop® plug-ins) can be used to remove any moiré that does appear in the final image.
Of course it may not be possible to remove all moiré in all cases, but in general an overall clean, sharp image with slight moiré is preferred over a soft focus image.

Moiré can happen with images from all digital cameras and scanners, but is more likely to happen with an SLR-type digital camera system because the lens, sensor and software are designed to produce the sharpest, most accurate image possible.

When reviewing images to see if moiré is present be sure to be looking at the image on the computer screen (or the camera's LCD) at the full, 100% view. If you zoom out on-screen a false moiré can be produced by the pattern of the monitor grill.