Let's Talk Contrast

amirm

Banned
Apr 2, 2010
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Seattle, WA
Simply put, contrast is the measure of dynamic range in a display. That is, the ratio of white to dark. Take a look at these two images:



The image on the right looks sharper, right? Well, it actually isn't! It is the exact same image with contrast boosted (too much so to make this point). In that sense, contrast ratio actually rates higher than resolution in perception of image fidelity. Indeed, sharpening an image in photoshop is sometimes called "local contrast enhancement."

The image on the right has higher contrast because there is larger differential between the black and white points. In a display the maximum is the amount of light the display can produce. The low level is how close it can get to putting out nothing.

We have taken an interesting journey in display technology. We had CRT sets for decades. While bulky and old fashioned, they excelled at one thing: black level. The phosphor was lit by bombarding it with electrons. Don't hit it with anything and you get total black since no energy will be emitted. In reality you would have reflections in a normally lit room from the glass but if you had a dark room, your blacks were essentially perfect. Maximum light though was a challenge as the CRT could not handle being driven too hard. Like an underpowered amp, CRTs clipped and bloomed when pushed.

We then moved to digital displays. Sadly while they can all get pretty bright relative to the CRT, they could not handle blacks as well. Years ago people would buy $20,000 flat panels that had contrast and hence picture quality that was far worse than the ancient technology CRT! Contrast was very low among other problems due to blacks not getting dark enough.

LCDs are a prime example of this. They have a backlight that generates light. So to get black, you need to filter it. Filtering down to nothing is very challenging and some light bleeds through. Plasma display is a bit like CRTs in the way it emits light but its cells have residual energy that increases their black level some. The solution for LCDs have been kludges in the form of modulating the back light. The image is analyzed and the level of black light changed either in small areas or large. Problem with that is that an image can simultaneously have bright and dark areas next to each other, down to two adjacent pixels. The result here would be that the backlight would either stay too high giving grayer blacks or light up too much showing halos.

At the other end, Plasmas like CRT cannot handle too much brightness across the screen image. Solution there is to pull down the brightness. Put a plasma next to an LCD in a store and watch the two handle something like snow. The plasma will pull power and brightness down, giving grayer whites than LCD.

Manufacturers keep working at these trying to improve the two end points. Meanwhile though, when they can cheat, which is always, they do to get better specs. There is no governance or definition for how contrast is spec'ed for the purposes of marketing. An LCD manufacturer then is free to pull this stunt: they feed the display white, and set the back light to 100% which in the case of an LCD is so bright that can give you a sun tan! Then they feed the set black, and set the backlight to minimum. Take the ratio and you get the million to 1 ratio. In real life you don't reach out for the black light control for every frame of video so clearly is not reality. Put a pattern on the screen that is alternating patches of black and white and the game is over. Contrast ratio can drop to 1000:1 or even lower! The latter of course is far closer to reality and is what magazine reviewers tend to measure using standardized tests.

Without exception, you want to ignore the manufacturer contrast ratio. It is that useless. It means absolutely nothing. The only way to learn that is by reading a magazine review that measures it.

There is more to this game. We don't just want the extremes but all the steps between. Displays can easily "crush" high levels of brightness or more commonly, low levels of darkness. Imagine a display that shows 10 for any levels of brightness from 0 to 10. This causes detail to be lost which is very noticeable in movies with darker scenes. If it is bad, and it used to be a few years ago, the image would turn into gray mush with no detail. Display manufacturers cheat, especially at shows, by using very bright material (beach scenes, studio shots of girls with lots of lighting, etc) so that this weakness is not seen. Fortunately much progress has been made in recent years to make displays perform a lot better.

BTW, here is the above image as I shot and processed it. I am in a very bright room so can't tell if it shows this point well but you should be able to see more detail in the dark trees relative to the top right image:



The image is from the Yellowstone park in winter by the way.
 

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