Back when I worked in broadcast television, the first thing they told the on-camera newscaster was “never wear a striped.” The skeptic who disregarded the message was shown his air-check the next day (along with a nasty note from management), and his jaw dropped when he saw the patterns of geometric lines crisscrossing his shirt. From that point forward, a solid-colored wardrobe was standard issue.
This was my first experience with the phenomena known as moiré, roughly defined as a visual interference pattern caused by two sets of parallel lines intersecting.
In the years before progressive scan HD, the NTSC standard ruled the land — and it’s still broadcast today. The NTSC system uses 525 horizontal scan lines to create video frames. When these parallel scan lines encounter another set of parallel lines (for example, on the newscaster’s shirt) via the video camera, visual havoc results.
A Few More Examples
We’ve all seen professional soccer matches on television. At most European stadiums, the field is surrounded by a solid ring of LED panels used for advertising and promotions. When the camera focuses on a player about to make a corner kick or an inbound pass, if the LEDs are visible in the background, serious moiré results — and the director usually cuts away from the shot pronto.
You’re at a concert with your trusty Nikon, and shooting the stage for the amazing lighting and fog effects. If there are LED panels anywhere on stage within your viewfinder, chances are you’re going to get moiré at almost every focal length.
You’re making a sales video for your company, and since you don’t have a scan converter handy, or a copy of Camtasia, you’ve got to shoot the monitor in order to capture the GUI. No matter if you shoot straight on or at an angle, there’s moiré galore — certainly more evident when you zoom in, yet it’s still there in your wide shots.
What gives? What can you do to prevent it?
Parallel Lines
Those two sets of parallel lines are the first problem. One set lives in your digital camera’s sensor (regardless of whether it’s a still camera or a video camera). The second set lives on the LED panels themselves. When those two sets interact, presto! When the lines are close together, you get fat moiré stripes. When those lines are farther apart, you get thin moiré stripes, repeating at regular intervals. If you shoot at an offset angle, you get repeating diagonal patterns of varying complexity.
The sensor inside your digital camera is, by default, an array of horizontal and vertical pixels. Some sensors are more “mega” than others — reaching up to 30 million pixels and above, with the chips packed at unimaginable nano densities. In addition, the better the camera lens, the better its ability to focus those pixels on a target image. Now, if you put that Nikon into video mode, you’re not using all the pixels on the chip, meaning that the horizontal and vertical lines are electronically farther apart. Keep this in mind as we explore deeper.
Pixels Times Three
By definition, an LED wall is comprised of horizontal and vertical stripes of the LED elements themselves. They’re not randomly mounted on the tiles — instead, they’re mounted with extreme precision. Regardless if the LED pitch (the distance between each discreet LED) is 20mm or 4mm, you can’t avoid the parallel lines. Step back into the concert arena, and your eye certainly can’t see the stripes — ah, but your camera’s sensor can.
So, Professor, on each LED wall, each individual LED accounts for one point in both a horizontal and a vertical line? Not so fast, Sparky. Inside the little case that encloses most modern LEDs, there are three individual LED elements that comprise the pixel — one red, one green and one blue. These are also mounted with precision, so in fact, you have additional “micro” rows of horizontal and vertical red, green and blue pixel elements.
Bayer Patterns
Now, with all this in mind, point your digital camera at an LED wall. Each pixel on the camera’s sensor is not an all-in-one RGB element. In fact, digital camera manufacturers lay out the sensor’s pixels in a special “Bayer” pattern, which uses four pixels (green, green, red and blue) to create one complete RGB pixel.
On a side note, be advised that when you purchase any digital camera that advertises some unbelievable amount of megapixels, they’re counting all the pixels on the sensor in the entire Bayer pattern. If you divide by four, you’ll get the real number of RGB pixels. For example, a 20 megapixel camera actually has five true RGB megapixels. Those marketing guys…
So the point is, on the LED wall, you have multiple sets of parallel lines, including the micro lines of the RGB elements within the individual LEDs. On the digital camera’s sensor, taking the Bayer pattern into account, you also have multiple sets of parallel RGB pixels. When you compose a shot and overlay those sets of lines on top of each other, you’re prone to get a joyful amount of moiré in the viewfinder — no matter if you’re standing directly in front of the screen, off-axis, in the mosh pit or at FOH.
Choices
So, Mr. Videographer, this is even worse than wearing a striped shirt. Should I give up and go back to film? Not quite.
In order to avoid moiré when shooting LEDs with a digital camera, you do have a few choices. An easy one is — don’t shoot the video wall. Compose your shots such that the framing avoids the LED walls entirely.
This is easy to say, but the sets aren’t designed with you in mind, Sparky. They’re designed for maximum audience impact and high-quality I-Mag, and it’s unlikely that the set designer will heed your requests to position the I-Mag stage left and stage right.
Better yet, learn a few techniques to avoid the problem entirely. Professional video cameras have “contour” controls that enable you to set the fine detail and sharpness of the image. Turn the contours down to soften the image. The crisper the image, the finer those parallel lines will focus, and the harsher the moiré. If you can accept a soft-focus look, the moiré will be reduced. (Back in the day, we placed nylon stockings over the camera lens to function as a soft-focus filter.)
Another tip — never leave the camera in auto-focus. If you do, the camera will certainly focus on the LEDs. In manual mode, however, if you focus on the talent (and the talent is located sufficiently in front of the LED panels on stage), the background will drop to soft focus and avoid the moiré.
Small and Large Apertures
This can also be enhanced by opening up the camera’s iris (aperture), letting your depth-of-field knowledge work to your advantage. The smaller the aperture, the deeper the depth-of-field, where objects both near and far are in focus simultaneously.
Conversely, a larger camera aperture yields a shallower depth-of-field, where objects are only in focus at one specific focal length — and all other focal lengths are out of focus. This in turn allows the talent to be in sharp focus, and the background to be soft.
Granted, these techniques work best if you’re shooting from on stage, or front and center near the stage. If you’re shooting from FOH or farther back, and you’re shooting wide to capture the full stage, chances are that the moiré will be reduced because the parallel lines blend together with little or no interaction.
Once you zoom in, however, all bets are off. In a future article, I’ll explore yet another problem that LED walls pose for the digital shooter — namely, color shift.
