Moire (pronounced more-ay) is an optical effect that appears when subjects with repeating patterns or lines of varying widths overwhelm the resolution capability of a camera sensor. It is often a source of frustration and confusion amongst photographers. The phenomenon is especially noticeable with high-resolution digital cameras that are capable of capturing a wide range of details.
The problem is most often caused by a pattern or line that cannot be resolved well by the camera’s sensor, which is functionally a grid. Depending on the subject, it can result in an intertwined pattern that looks like a shimmering oil slick or a rainbow. It is often seen in photographs of woven fabric with fine detail or closely arranged patterns, such as bird feathers. It is also common in photographs of laptop and computer screens and televisions that use a patterned screen.
There are various techniques to reduce moire, most of which involve using a filter in front of the sensor to “break up” the light and eliminate the interference. However, a more reliable solution is to choose a camera that is designed not to suffer from this issue. For example, the new Sony A7 series of full-frame cameras utilize a back-illuminated sensor design to avoid moire without the need for an OLPF. This sensor design uses a unique pixel array to scatter RGB pixels to reduce false colors and moire patterns.
Another method of reducing moire is to adjust the camera’s aperture settings before taking a shot. This can be achieved by starting with the widest aperture setting and lowering it to a more appropriate number, such as f/4 or f/5.6, if necessary. Increasing the shutter speed to reduce the amount of light also helps, although it may introduce additional blurring to the image that is undesirable.
When shooting in conditions that are likely to produce moire, it is always a good idea to examine the image at 100% or higher on the camera LCD to look for any distortions or unusual coloration. Likewise, it is a good idea to review the image at lower resolutions, such as thumbnails, to see if it shows any of these patterns.
Moire patterns can sometimes appear in photos even when the quality of the photo is excellent. This can happen when the images are compressed, for instance when they are used on LinkedIn profile photos or when they are reduced to a smaller file size. The moire can show up as yellow, green, magenta or purple tints and colors that are almost rippled. They can be reduced or eliminated by using one of the methods discussed in this article or by post-processing the image in Photoshop or Lightroom. The latter has a specific tool for removing moire, which can be found in the Develop module under the Adjustment Brush icon. To find it, first make sure that all other sliders in the Develop module are set to 0.