HOW TO MANAGE THE MOIRÉ EFFECT FOR LIVE PRODUCTIONS

Moiré is the result of an optical distortion caused by the overlay of two grids of lines that have different sizes or angles. This interference pattern creates a visual distortion effect. 

A common question is why Moiré is more prevalent in video cameras than in film. The grains in a film emulsion frame are randomly arranged, while all the photosites in video cameras are in a regular grid pattern. Regardless of whether it’s a rolling shutter, global shutter, CMOS, CCD, single-chip, or three-chip, the photosites form a grid, much like a screen. Thus, capturing it through another screen or a series of parallel lines that resolve at a similar frequency introduces the possibility of the Moiré Effect appearing. 

Camera systems rely on the fact that as frequencies increase, regular patterns like fences, screens, stripes, bricks, tiles, and fabrics become smaller and tend to lose contrast (measuring contrast from dark to light is modulation depth, and the modulation depth vs. frequency graph is the modulation transfer function of an optical system like a lens). Modulation depth usually decreases as patterns become smaller (frequencies increase) and tends to zero when intricate details are no longer resolved. Lens defects or fog in the air add to the abruptness of the modulation depth decrease. 

LED walls have properties that make them bright and sharp, presenting a challenge for Moiré. Individual LEDs are point sources that do not blend with each other. They are emitters, meaning a dark pixel can be completely turned off, and often there is a black matrix between LEDs (like another screen). The contrast between pixels is extreme (high modulation depth) and very robust as pixels become smaller since each LED generates its own bright light, resulting in minimal loss of modulation depth. Therefore, LEDs appear smaller as they approach the pixel frequencies of the image generator. 

When pixels are resolved in an image, they are just small points covering many photosites in the image generator, and there is no Moiré. When pixels are not resolved at all, they appear as a flat color, and there is no Moiré. Problems occur in the middle, at nodes where the camera resolution is a small multiple of the pixel size or vice versa, close enough to interfere with each other. These nodes can be easily observed during zoom. To avoid the Moiré Effect, the OLPF creates a boundary where pixels are blurred. This boundary is reached more quickly when the LED wall is out of focus. Lens depth-of-focus properties can greatly help in this case. Depth-of-focus is the range of distance within which an object will appear in focus. Cinematic-style shooting takes advantage of reduced depth-of-focus to focus between subjects with sharp or soft focus. Larger image generators along with longer focal lengths and open apertures lead to a smaller depth-of-focus. Zooming to longer focal lengths (closer) increases the resolution of LED pixels but simultaneously decreases the depth-of-focus, causing the LED wall to blur when a lens zooms in, provided the lens is focused on a plane far from the LED wall.

Broadcast system cameras usually have ND filter wheels, which are important for using a wide aperture to reduce depth-of-field. All Panasonic studio cameras (HC3900, UC3300, PLV100, and UC4000) use motorized ND filter wheels for quick access to ND filter adjustments in the control room.

The Panasonic UC4000 has a dual filter wheel. The first is the motorized ND filter wheel, and the second is known as the effects filter wheel, traditionally including color correction filters, a cross or star effect filter, and a diffusion filter. Panasonic offers a modification where the diffusion filter is replaced with a custom OLPF that, when activated, adds an additional high-frequency screen before the image sensor, reducing the chances of Moiré. This OLPF causes a slight reduction in sharpness in UHD images but almost no change when recording in HD. The modified camera model designation is MOD5AK-UC4000, and it can be custom-ordered from Panasonic.

The Panasonic AK-UC4000 and AK-UC3300 studio cameras produce raw image data sent to the Camera Control Unit (CCU) for processing. This CCU has a more powerful FPGA that can reduce the Moiré Effect or make it less noticeable. Electronic Moiré reduction is limited because, as mentioned earlier, Moiré is an optical phenomenon.