Levels: The Key To Contrast And Detail
We’ve established that dynamic range is the most important element in image quality. Just like audio, the greater the difference between the extremes (soft and loud, light and dark), the greater the realism and sense of depth. And in a rare bit of good fortune, this is the one thing a user can achieve with a monitor that doesn’t require a meter or fancy software! What we’re talking about are black and white levels, or, as they’re more commonly known, brightness and contrast.
At some point in history, a not-so-clever television engineer decided that the controls affecting dynamic range should be called brightness and contrast rather than black level and white level. This has created confusion as to what these adjustments are actually for. It’s actually quite simple, though. Brightness is black level and contrast is white level.
Now, what are we doing when we change those controls? Black level/brightness refers to the minimum level of light a display will produce. White level refers to the maximum level of light. Obviously, by minimizing black and maximizing white, you'll achieve the highest contrast ratio and the greatest dynamic range possible for a given display. The trick is to set the levels properly, while retaining all of the image's detail.
To illustrate the importance of setting black and white levels correctly, let’s check out an actual photograph. We modified the original in Photoshop by using the Levels dialog to manipulate the black and white levels. This is functionally the same as adjusting the brightness and contrast controls on your monitor. No adjustments were made to color.
Here’s a shot of singer Gavin Rossdale performing with Bush.
This is a pretty detailed photograph. You can clearly see the definition in his arms, his hair, and his jeans. Plus you can see the audience far in the background. And check out the subtle outline of his right hand against the guitar’s body.
Here’s the histogram.
Nearly the entire brightness range is represented, except for a few steps at the dark end. And there are one or two steps at the bright end that get crushed. This image is straight from the camera with no post-processing.
Here’s the same shot with the black level set too low.
The audience and Gavin’s hair get obliterated. His right hand now blends into the guitar’s pick guard. And his jeans have far fewer wrinkles than before. This is what we mean when we use the term crush or clip in reference to black. The darkest information is crushed together and shadow detail is lost. Where there were perhaps 50 gradations of black, there is now only one. While an artist may purposely modify an image like this for extra impact, the fact is that some of the original information was lost.
Now let’s see what happens when you set white level too high.
Gavin’s face and right arm turn into formless white shapes, and we lose the dimension we saw in the original. The many shades of white that created detail in his face are now blended together. Since the audience in the background is also brighter, the sense of depth is significantly reduced. It’s harder to tell just how far away they are.
Here’s the same photo with the black level set too high.
You can still see the detail, but now there’s a hazy look to the photo, as if a filter was put in place. Any sense of image depth is drastically reduced.
Our final example shows the effect of setting the white level too low.
Again, the detail is all there, but the photo looks dim and underexposed. This and the previous image retain all of their detail. However, by reducing the dynamic range, the vibrancy and impact are significantly lower.
We’ll show you how easy it is to fix these problems with a couple of test patterns you can download from the Internet. Setting black level and white level properly doesn’t require instruments or software. And it’s one of the best things you can do to improve your monitor’s image. By maximizing dynamic range, you’re well on your way to better pictures!
To fully take advantage of a display’s dynamic range, we also need to understand what’s happening to the points in between black and white. This is where the discussion turns toward gamma.