LED screens are nothing new to filmmaking; so-called "rich mans process" has been used for years to simulate car travel in driving scenes. But their usage in virtual production truly unlocks their potential in the filmmaking process.
However, with great power comes great complexity, and there are a lot of things about shooting LED screens that are counter-intuitive. Unfortunately, these issues aren't easily fixed in post--so it'd important to get them right in-camera. In this post, I'll go over a few of the most critical things to consider when shooting on an LED backdrop.
1. Exposure is critical
Proper exposure is a huge part of making the image feel real, and not artificial. It's best to think of an LED screen (or any screen) as the opposite of a camera--just as a camera "sees" a certain amount of dynamic range, a screen can display a certain amount. This is referred to as the screens contrast ratio. The higher the contrast ratio, the more dynamic range information the screen displays to camera. Make sense?
To that end, it's important to not think of the screen brightness as a "tool" and instead think of the image on screen as "reality." You shouldn't be varying the brightness of the screen to match your exposure, you should be aiming to get the brightness of the image to be as close to reality as possible, and lighting/exposing just as you would in that real situation.
This means that in bright situations, the screen will be very bright--that's fine. Compensate with ND filters just as you would in real life, and that will provide the most realistic image. Remember, cameras don't see light or color linearly, and their particular look and color science is dependent on the intensity of the light hitting the sensor. Trying to aim to empirical reality will trick the camera into thinking that the background is real.
2. Balance of foreground and background
To add onto exposure, it's very important to match the background and your foreground subjects with both color and intensity of light. LED screens can't get as bright as the sun, or as a specular off a car, so it's critical to compensate with that in your exposure. It's very common to see amatuer LED wall footage where the backlight of the subject is brighter than the sun in the background. False color is your best friend in matching highlights/shadows!
The same applies for the shadows as well. An LED wall shoot can be instantly given away by an elevated black level on the screen relative to the foreground subjects. Remember--under studio lighting conditions, an LED wall will only provide about 10 stops of dynamic range information. You must consciously choose where to place that information on the cameras response curve.
A trick for this is to over-rate or under-rate the cameras ISO. If you're shooting a bright outdoor scene for real, most of the information will be above the toe of your cameras response, I.E. most information will be in the highlights. This is where using relative lighting ratios can get you into trouble, as the way the camera sees information in the shoulder is different that how it sees the same information in the toe. Over rating or under rating your camera can push the information more towards the correct place on the exposure curve.
3. LED walls hate dim/low contrast scenes
LED walls are digital, and as such, they have a particular bit depth. Even though most of them will be wired for 10 bit, that still may not be enough to eliminate artifacts on very low contrast or dim scenes--there just aren't enough bits of information available to represent incredibly fine tonalities.
If you're planning on shooting a very low key scene, consider shooting everything a stop over and bringing the image back down in the grade. This will help the LED wall display the best image it can for that scene.
4. Flag your lights!
LED walls are made of plastic, and as such are quite reflective. Even small amounts of ambient light in the room will reflect off the screens and raise their black levels, reducing contrast or creating hot spots. This results in images that feel "thin" and artificial, even if everything is calibrated correctly.
It's absolutely critical to tease, flag, and grid off all ambient light spill from the screens. Your LED stage will have a quick way to black out the screens when asked, allowing you to see exactly where light is hitting the screen. Because LED walls are made of faceted flat tiles, curved screens can be very problematic as shallow angle lights can highlight the faceted nature of the wall.
What tips and tricks have you found to be effective? Comment below!