Pattern shown in LED screen background used as backdrop on a stage

Question

I'm trying to figure out an oddity in photographing a large LED background. I'm using a Canon 5D Mark II with a 100-400 lens although it occurs with any lens and at any focal length. The camera is picking up a pattern that isn't visible with the naked eye.

It looks like aliasing that you might get when photographing a LED monitor except here the screen is the size of a billboard. I'm not sure I would call it a moire pattern though.

The shutter speed must be 250 minimum, usually 320 or 400 due to action on stage requiring it otherwise action is blurry. Lighting is a bit low so ISO 3200 has to be used and aperture is wide open either 2.8 or 4.0 depending on the lens.

Obtaining a shallow depth of field can help by blurring the background a bit but it's still there. I haven't really noticed a difference in the angle I'm photographing as to if it makes a difference but a straight on shot is what is needed in most cases to capture the scene. A much slower shutter like 160 starts reducing the effect but it is too slow for action in the scene.

Using my Iphone 7 Plus, taking the same photo all lines are completely gone! The picture is 100% solid. That's using an Iphone 7 plus and an automatic photo (no settings). The only time the iphone 7 plus does not do well is if the action is in high motion, the action may be a bit blurry.

The only time I have been able to obtain a good photo with the Canon 5D Mark II (also tried a Sony A6000) and multiple lenses is when using a flash but a flash isn't really acceptable to fire off during a show. It appears with the flash firing it is indicating a shutter of 1/200 (which is usually too slow for action) but how the flash works I believe it is only firing for a fraction of the 1/200 even though I have it manually set for a faster shutter. The action is frozen and the screen is solid. Shooting at 1/200 without the flash yields the same results of lines in the screen. So I'm not exactly sure how the flash is causing the LED screen to appear solid but it works. The iphone 7 plus (no flash) works.

So how do I shoot in order to remove the lines in the photo of the LED screen?

Thanks... Gary

Answer 1

There are probably at least two things going on here that are contributing to the artifacts you see in your photos:

• Moire. When you take a picture of a regular pattern (like the rows and columns of a large LED display screen) the pattern you are photographing and the pattern on your camera's digital sensor can interfere with one another. It's like looking through two screen doors, one in front of the other, at the same time. The way the two patterns overlap when lined up creates the odd patterns we call moire.

Before you throw your DSLR away, be sure the pattern is showing up when you view your images at full resolution (i.e. at 100% where one screen pixel equals one pixel in the photo). Often moire can be caused by the scaling errors of reducing a high resolution image with repeating patterns in it to a lower resolution image for viewing. If you are viewing your 5D Mark II's 5616 x 3744 21MP images on the camera's 920K dot (just less than 1MP) rear LCD or on a typical computer monitor then the images are being scaled down significantly. Check a small area of the on-stage screen by pixel peeping it at 100% and see of the pattern is still there.

• Flicker/Modulation. LEDs work by flickering on and off very rapidly. Large display screens that use LED lights control the perceived brightness of each color of each pixel by modulating how long each LED bulb is on and how long it is off. This modulation is done at a fairly high frequency, but when you are shooting at a shutter time that is faster/shorter than one complete cycle of the modulation you will catch the screen with some of the lights pulsed on for the entire exposure, some pulsed off for the entire exposure, and some that pulsed on or off while the shutter was open. This can also create strange patterns that look a lot like moire.

The solution to flicker/modulation, as you have discovered, is to lengthen the shutter time until it is slow enough to average out the on/off pulses of each LED bulb in the display on stage. Of course that also means your rapidly moving subjects are now blurry from their motion.

Moire, and to a lesser extent pulse width modulation, can be dealt with in postprocessing with varying degrees of success. Doing selective sharpening by using a brush to select and reduce the sharpening of the LCD display in the photo can help smooth it out. You can control the sharpening individually for each viewing size/resolution and create, optimize, and export different resolutions of the image for different display environments. This can also help to reduce moire caused by scaling errors.

With regard to the effect of using flash:

• If the flash is being used in E-TTL mode with the camera in Av exposure mode or with Auto ISO selected the overall exposure of the ambient light might be getting reduced by the camera so that the influence of the light emitted by the LEDs in the screen is less in terms of the overall light in the photo.
• Since the sync speed is 1/200 the shutter time is also longer than when shooting at 1/320 or 1/400 without the flash. The rest of the subjects onstage are being "frozen" by the short duration of the flash. But the LED screen on the stage is emitting its own light for the entire time the shutter is open.
• The light from the flash may be getting into the screen and bouncing around inside and then reflecting back to the camera which would fill in the dark gaps between each bulb and smooth out the moire from the pattern of the screen's bulbs.

Answer 2

This looks like moire. It basically happens because the resolution of your camera is close to that of the pattern on the stage background. Usually you can work around it by increasing resolution (switching to a 5DS for example) or lowering it. You can artificially lower the resolution of your camera by using an aperture which is sub-optimal, either very wide when lenses are often softer or beyond the diffraction limit.

For more about why moire occurs, see this question.