Crushed Blacks: Dealing With Detail Loss in Dark Scenes
Something I see a lot of with video surveillance is a complete lack of detail in the dark regions of a scene. This need not be at night but can happen when part of the scene is overexposed and sometimes for no apparent reason at all. What do you think is the cause of this loss of detail and how can it be rectified?
It sounds like crushed blacks – essentially the black point of an image is being shifted so the dark regions of the scene are all represented as being devoid of light – and thus stripped of detail. It’s important to bear in mind here that crushed blacks are not about tone but about detail. This may be caused by engineering, improper camera settings, or poor monitor calibration. Blacks may also be crushed in software to enhance the appearance of other parts of the scene, reducing the appearance of over exposure and increasing contrast and saturation.
During our tests, we notice some cameras do relatively well with challenging light levels in variable scenes but others exhibit issues with crushing blacks. Some cameras seem to default to crushing blacks when some settings are selected. It’s worth bearing in mind most CCTV cameras are about increasing wide dynamic range, not decreasing it – that means many cameras are unlikely to crush blacks at default. If you believe a camera is crushing blacks to expose for the bright part of the scene at the expense of the dark parts, apply WDR.
You’ll see the appearance of the image change noticeably as blacks are lifted – this lifting relates only to image tone, not to detail. You’ll see an overall lightening, brightening and warming of the entire scene. Sometimes this change can be too much – try applying back light compensation instead of WDR – it’s less intrusive and may retain better image balance if backlight is not too extreme.
Excessive crushing of blacks is not just an in-camera software issue. When video cards convert signals, they will always introduce some distortion of greyscale range and unless you know how much you can’t calibrate to correct it. I think it’s also fair to acknowledge that crushed blacks are also a feature of poor quality and poorly calibrated monitors. Most monitors are never calibrated properly, or are calibrated once and left to drift.
Something else to take into account is that older flat monitors all suffered badly from crushed blacks no matter what their calibration. Many old monitors are not equipped to allow proper calibration and given the fact many security departments use screens until they fall off the wall, the presence of crushed blacks (loss of detail in dark areas, not tone) is pretty much guaranteed.