Levels is one of the quickest, most effective ways to adjust image tonality. But, if care isn't taken, it can cost you valuable image data.
Here we see a typical scan of a faded photo. As you can see from the histogram, some highlight detail has already been clipped. We're going to use Levels to adjust the shadow values without losing any additional detail.
The first impulse with levels is to grab the sliders and move them back and forth until the image "looks right". As we'll see in the next steps, this can be a costly mistake.
Instead, hold down the Alt key before moving the shadow slider. As you can see here, the image display is all white, which means no image detail will be lost with this setting.
Still holding down the Alt key, continue moving the shadow slider towards the right. You'll begin to see image data showing through. This is valuable data that will be clipped and lost if you were to accept these settings.
While still holding the Alt key, continually move the shadow slider until the image detail is just barely gone. This guarantees you'll use the maximum possible tonal range without sacrificing any image detail.
And here's our final, adjusted image.
This works just as well with the highlight slider, but has no effect at all on the midtone slider.
And remember: always adjust levels using an adjustment layer!