Have you heard the term “black crush” while looking at TV or monitor reviews? The problem is getting more attention now that display manufacturers are trying to improve black reproduction, particularly on LED-lit LCD models. So what are “crushed blacks,” and do you need to worry about them?
Black Crush Means a Loss of Shadow Detail
Black crush refers to the loss of fidelity in especially dark areas of an image. The term might apply in both the photography and video spheres but is most commonly used to describe the loss of shadow detail in moving images like movies and games.
You might not immediately notice black crush until you see the “correct” image displayed alongside it (for example, on a different display that’s been correctly calibrated). You can see a simulated example in the image below, where the right side has lost detail in the bricks under the shadow. In other words, this image has “crushed blacks.”
The problem is unlikely to make content unwatchable, but it does detract from the overall presentation. In movies, you might miss subtle details around the edge of the frame, while the issue can make it way too difficult to see what’s happening in some games (particularly in brightly lit rooms).
There are all sorts of reasons that black crush can happen, and not all of them are down to the display. If the shadow detail wasn’t captured in the first place because the camera wasn’t set up to do so, blacks will appear crushed. Some directors and photographers use this technique to intentionally create negative space.
Your TV or Monitor Makes a Difference
All too often the problem lies with the display or source device (like a games console). Many games require the player to calibrate gamma and white point when the software is first launched, and setting this up incorrectly (or getting it wrong at a system level) might cause a loss of shadow detail. Sometimes games implement HDR poorly, also causing black crush.
Most consumer displays are never calibrated when they leave the factory, and without proper calibration by a professional, they will always produce an image that deviates from the source. This is why content creators and photographers are strongly encouraged to use a calibrated display when editing their work.
Sometimes, the TV really is the problem. OLED is a self-emissive display technology, which means that the pixels can be turned off to display “true” black. Unfortunately, OLED also has difficulty coming out of black, which on some models can lead to a loss of shadow detail as the TV struggles to reproduce the subtle tones that exist between the “on” and “off” states at a pixel level.
Many LED-lit LCD TVs now use dimming algorithms to turn off or reduce the light behind dark or black scenes. This helps the TV produce a much deeper black level, but it almost always comes at the expense of shadow detail. Generally speaking, the more dimming zones a display has, the less severe the issue will be.
How to Test Your Own Display for Black Crush
An easy way to test for black crush is to use a starfield test. On a display that exhibits black crush, many of the stars won’t be visible. On an OLED, the vast majority of stars should be visible, since a bright white pixel can sit alongside a pure black one without any dimming algorithm interfering with the image.