Why is it black when shooting TV with a camera?

Simply put, moire is a manifestation of the principle of beat. Mathematically speaking, when two sine waves with similar frequencies are superimposed, the amplitude of the synthesized signal will change according to the difference of the two frequencies. Beat frequency principle is widely used in radio, television and communication for frequency conversion and modulation.

Similarly, the beat principle also applies to spatial frequency. The superposition of stripes with slightly different spatial frequencies will gradually shift and form beats due to the difference of stripe spacing and overlapping position.

If the spatial frequency of CCD(CMOS) pixels is close to the spatial frequency of fringes in the image, moire fringes will appear. In order to eliminate moire fringes, the resolution of the lens should be much smaller than the spatial frequency of the photosensitive element. If this condition is satisfied, it is impossible to have fringes similar to photosensitive elements in the image, so moire fringes will not appear. In some digital cameras, in order to reduce moire fringes, a low-pass filter is installed to filter out the parts with high spatial frequency in the image, which will of course reduce the clarity of the image. In the future, if the pixel per inch can be greatly improved, far exceeding the resolution of the lens, moire fringes will not appear.

Reduce and eliminate the influence of moire fringe in photography

1, change the camera angle. Because the angle between the camera and the object will lead to moire fringes, any moire fringes can be eliminated or changed by slightly changing the camera angle (by rotating the camera).

2. Change the position of the camera. In addition, moire fringes can be reduced by changing the angular relationship by moving left and right or up and down.

3. change the focus. Too clear focus and height details on detail patterns may lead to moire fringes, and a slight change in focus can change the clarity, thus helping to eliminate moire fringes.

4. Change the focal length of the lens. Different lens or focal length settings can be used to change or eliminate moire fringes.

5. Use software to process. Such as Nikon capture or Photoshop plug-in, to eliminate any ripple on the final image.