Using the Primary Contrast Controls

The primary contrast controls consist of three vertical sliders that are used to adjust the black point, the distribution of midtones, and the white point of the image.

Figure. Primary contrast sliders.

Each slider is a vertical gradient. Dragging down lowers its value, while dragging up raises its value. A blue bar shows the current level at which each slider is set, while the third number in the Output display (labeled L) below each color control shows that slider's numeric value. Contrast adjustment is a big topic. For more information, see:

Using Contrast Sliders with a Control Surface

In the Primary In, Secondaries, and Primary Out rooms, the three contrast sliders usually correspond to three contrast rings, wheels, or knobs on compatible control surfaces. Whereas you can adjust only one contrast slider at a time using the onscreen controls with a mouse, you can adjust all three contrast controls simultaneously using a hardware control surface.

When you’re using a control surface, the Encoder Sensitivity parameter in the User Prefs tab of the Setup room lets you customize the speed with which these controls make adjustments. For more information, see Control Surface Settings.

Adjusting the Black Point with the Shadow Slider

The behavior of the Shadow contrast slider depends on whether or not the Limit Shadow Adjustments preference (in the User Prefs tab of the Setup room) is turned on. (For more information, see User Interface Settings.)

  • If Limit Shadow Adjustments is turned off: Contrast adjustments with the Shadow slider are performed as a simple lift operation. The resulting correction uniformly lightens or darkens the entire image, altering the shadows, midtones, and highlights by the same amount. This can be seen most clearly when adjusting the black point of a linear black-to-white gradient, which appears in the Waveform Monitor as a straight diagonal slope. Notice how the entire slope of the gradient in the Waveform Monitor moves up.
    Figure. Examples of limit shadow adjustment turned off.
  • If Limit Shadow Adjustments is turned on: The black point is raised, but the white point remains at 100 percent. This means that when you make any adjustments with the Shadow contrast slider, all midtones in the image are scaled between the new black point and 100 percent. Notice how the top of the slope in the Waveform Monitor stays in place while the black point changes.
    Figure. Example of limit shadow adjustment turned on.

You'll probably leave the Limit Shadow Adjustments control turned on for most of your projects, since this setting gives you the most control over image contrast (and color, as you'll see later) in your programs.

Contrast adjustments to the shadows are one of the most frequent operations you'll perform. Lowering the blacks so that the darkest shadows touch 0 percent (seen in the bottom of the Waveform Monitor's graph or on the left of the Histogram's graph when either is set to Luma) deepens the shadows of your image. Deeper shadows can enrich the image and accentuate detail that was being slightly washed out before.

Figure. Before and after lowering shadows.

Lowering the blacks even more, (called crushing the blacks because no pixel can be darker than 0 percent), creates even higher-contrast looks. Crushing the blacks comes at the expense of losing detail in the shadows, as larger portions of the image become uniformly 0 percent black. This can be seen clearly in the black portion of the gradient at the bottom of the image.

Figure. Example of crushed blacks in image and waveform.

Note: Even if Limit Shadow Adjustments is turned on, you can still make lift adjustments to the image using the Master Lift parameter in the Basic tab. See Master Contrast Controls.

Adjusting the Midtones with the Midtone Slider

The Midtone contrast slider lets you make a nonlinear adjustment to the distribution of midtones in the image (sometimes referred to generically as a gamma adjustment). What this means is that you can adjust the middle tones of the image without changing the darkness of the shadows or the lightness of the highlights.

Here are two examples of using the Midtone contrast slider. The midtones have been lowered in the following image. Notice how the overall image has darkened, with more of the picture appearing in the shadows; however, the highlights are still bright, and the shadow detail has not been lost. The top and bottom of the gradient's slope in the Waveform Monitor remain more or less in place, and the slope itself curves downward, illustrating the nonlinear nature of the adjustment.

Figure. Before and after lowering the midtones.

Next, the Midtone slider is raised. The image has clearly lightened, and much more of the picture is in the highlights. Yet the deepest shadows remain rich and dark, and the detail in the highlights isn't being lost since the highlights are staying at their original level. Again, the top and bottom of the gradient's slope in the Waveform Monitor remain more or less in place, but this time the slope curves upward.

Figure. Before and after raising the midtones.

No matter what contrast ratio you decide to employ for a given shot, the Midtone slider is one of your main tools for adjusting overall image lightness when creating mood, adjusting the perceived time of day, and even when simply ensuring that the audience can see the subjects clearly.

Note: Even though midtones adjustments leave the black and white points at 0 and 100 percent respectively, extreme midtones adjustments will still crush the blacks and flatten the whites, eliminating detail in exchange for high-contrast looks.

Adjusting the White Point with the Highlight Slider

The Highlight slider is the inverse of the Shadow slider. Using this control, you can raise or lower the white point of the image, while leaving the black point relatively untouched. All the midtones of the image are scaled between your new white point and 0 percent.

If the image is too dark and the highlights seem lackluster, you can raise the Highlight slider to brighten the highlights, while leaving the shadows at their current levels. Notice that the black point of the gradient's slope in the Waveform Monitor remains at 0 percent after the adjustment.

Figure. Before and after raising the white point.

Note: In this example, Broadcast Safe has been turned off, and you can see the white level of the gradient clipping at the maximum of 109 percent.

If the highlights are too bright, you can lower the Highlight slider to bring them back down, without worrying about crushing the blacks.

Figure. Before and after lowering the highlights.

Overly bright highlights are often the case with images shot on video, where super-white levels above the broadcast legal limit of 100 percent frequently appear in the source media (as seen in the previous example). If left uncorrected, highlights above 100 percent will be clipped by the Broadcast Safe settings when they're turned on, resulting in a loss of highlight detail when all pixels above 100 percent are set to 100 percent.

Figure. Before and after clipping highlights above 100 percent.

By lowering the white point yourself, you can bring clipped detail back into the image.

Note: Values that are clipped or limited by Color are preserved internally and may be retrieved in subsequent adjustments. This is different from overexposed values in source media, which, if clipped at the time of recording, are lost forever.

While modest adjustments made with the Highlight slider won't affect the black point, they will have an effect on the midtones that is proportional to the amount of your adjustment. The influence of the Highlight slider falls off toward the shadows, but it's fair to say that adjustments made with the Highlight slider have a gradually decreasing effect on approximately the brightest 80 percent of the image.

Figure. Interaction between highlights and midtones adjustments.

For this reason, you may find yourself compensating for a Highlight slider adjustment's effect on the midtones of your image by making a smaller inverse adjustment with the Midtone slider.

The suitable white point for your particular image is highly subjective. In particular, just because something is white doesn't mean that it's supposed to be up at 100 percent. Naturally bright features such as specular highlights, reflected glints, and exposed light sources are all candidates for 100 percent luma. (Chances are these areas are at super-white levels already, so you'll be turning the brightness down if broadcast legality is an issue.)

On the other hand, if you're working on an interior scene with none of the previously mentioned features, the brightest subjects in the scene may be a wall in the room or the highlights of someone's face, which may be inappropriately bright if you raise them to 100 percent. In these cases, the brightness at which you set the highlights depends largely on the kind of lighting that was used. If the lighting is subdued, you'll want to keep the highlights lower than if the lighting is intentionally bright.

Expanding and Reducing Image Contrast

For a variety of reasons, it's often desirable to stretch the contrast ratio of an image so that it occupies the widest range of values possible, without introducing unwanted noise. (This can sometimes happen in underexposed images that require large contrast adjustments.)

Most images don't start out with the highest-contrast ratio possible for the shot. For example, even in well-exposed shots, video cameras often don't record black at 0 percent, instead recording black levels at around 3 to 4 percent. For this reason alone, small adjustments to lower the black point often impress without the need to do much more. In other cases, an image that is slightly over or underexposed may appear washed out or muddy, and simple adjustments to lower the darkest pixels in the image and raise the brightest pixels in the image to widen the contrast ratio have an effect similar to “wiping a layer of grime off the image” and are often the first steps in simply optimizing a shot.

Figure. Before and after expanding image contrast.

In other cases, you may choose to deliberately widen the contrast ratio even further to make extreme changes to image contrast. This may be because the image is severely underexposed, in which case you need to adjust the Highlight and Midtone sliders in an effort to simply make the subjects more visible. You might also expand the contrast ratio of an otherwise well-exposed shot to an extreme, crushing the shadows and clipping the highlights to create an extremely high-contrast look.

Figure. Clipping highlights.

Important: When you expand the contrast of underexposed shots, or make other extreme contrast adjustments, you may accentuate film grain and video noise in the image. This is particularly problematic when correcting programs that use video formats with low chroma subsampling ratios. For more information, see Chroma Subsampling Explained.

Of course, you also have the option to lower the contrast ratio of an image. This might be done as an adjustment to change the apparent time of day (dulling shadows while maintaining bright highlights for a noon-time look) or simply as a stylistic choice (lighter shadows and dimmer highlights for a softer look).

Figure. Before and after lowering image contrast.

What Exactly Is Image Detail?

Image detail is discussed frequently in this and other chapters, mainly within the context of operations that enhance perceived detail, and those that result in the loss of image detail. Simply put, image detail refers to the natural variation in tone, color, and contrast between adjacent pixels.

Because they occur at the outer boundaries of the video signal, the shadows and highlights of an image are most susceptible to a loss of image detail when you make contrast adjustments. This results in the "flattening" of areas in the shadows or highlights when larger and larger groups of pixels in the picture are set to the same value (0 in the shadows and 100 in the highlights).

It's important to preserve a certain amount of image detail in order to maintain a natural look to the image. On the other hand, there's no reason you can't discard a bit of image detail to achieve looks such as slightly crushed blacks, or widely expanded contrast for a "high-contrast look" with both crushed blacks and clipped whites. Just be aware of what, exactly, is happening to the image when you make these kinds of adjustments.

Contrast Affects Color Balance Control Operation

There's another reason to expand or otherwise adjust the contrast ratio of an image before making any other color corrections. Every adjustment you make to the contrast of an image changes which portions of that image fall into which of the three overlapping tonal zones the color balance controls affect (covered in Using Color Balance Controls). For example, if you have a low-contrast image with few shadows, and you make an adjustment with the Shadow color balance control, the resulting correction will be small, as you can see in the following gradient.

Figure. Low contrast image with Shadow color adjustment.

If, afterward, you adjust the Shadow or Midtone contrast sliders to lower the shadows, you'll find more of the image becoming affected by the same color correction, despite the fact that you've made no further changes to that color control.

Figure. The same Shadow color adjustment with altered contrast changes the result.

This is not to say that you shouldn't readjust contrast after making other color corrections, but you should keep these interactions in mind when you do so.