When taking pictures of intricate repetitive patterns such as grid lines, embroidery, fine cloth or other geometric shapes (such as when taking pictures of architecture with numerous columns or pillars) one can face the problem of moiré. It is an unintentional pattern of lines and shapes that transposes on the final pictures as a result of the repetitive geometrical patterns that the camera sensor cannot discern perfectly. Moiré effects are caused when the camera sensor adds the second pattern (in addition to the first and the primary pattern which is the subject of the picture). The sensor then confuses between the two patterns and moiré is created.
To have a deeper look into why the sensor creates moiré we have to take a look into a camera sensor and its architecture. Modern digital cameras have sensors which have replaced the film camera systems of the yesteryears. The sensor has numerous tiny light sensitive photodiodes. Each photodiode can be compared with a chamber that captures light. However they can capture only a single of the primary three colors (i.e.; red, blue and green). The reason is if the photodiodes are sensitive to all the colors they will not be able to distinguish between each. A filter placed on top of each chamber is what helps in capturing one color and in effect keeping out the remaining colors. Arrays of photodiodes thus capture the three primary colors according to the filter on the top. There are several patterns used to capture light and the most popular is the Bayer Array. It has an alternating array of chambers and filters which is designed to capture 2 green light waves for every red and blue light waves. The reason is that the human eye (and therefore the camera, as it is designed very much on the same principle) is more sensitive to the green color. This overall pattern of red, green and blue is what creates the moiré.
The optical low pass filter (OLPF) is a filter that is used in state of the art camera technology to ensure that moiré (and with that an additional problem of false color) can be eliminated. Nikon uses its OLPF in all of its current DSLR bodies, which is why there are no discernable moiré or false coloring in the pictures produced by the cameras. It is pertinent to note that using the OLPF can sometimes soften the picture. one can identify this with the loss of sharpness in the final pictures. Studio photographers when they are shooting RAW and does not counter the problem of moiré or false color may not like to use the OLPF. They prefer the images to stay sharp even when making a tight crop. A recent set of camera systems can be taken as a final evaluation to explain how the presence and absence of OLPF can affect the pictures.
The D800 and D800E are two camera systems that Nikon has recently launched. The first one has an OLPF and the second one does not use the same. The result is no false coloring and moiré in the D800 but the images are softer compared to the D800E which does not use the same filter.
Photographers who have used both the cameras are however divided with those preferring to exploit the 36 megapixels for tighter cropping and sharper resolution at that levels going for the D800E.