Levels and Curves: Photoshop’s Left and Right Ventricles Pump Luminance

Simply put, the Levels and Curves adjustments allow you to control the luminance of an image on a channel-by-channel basis. With these features, you can correct brightness, contrast, and color casts. They are essential commands and the two most powerful color correction facilities in Photoshop. The Levels command offers relative simplicity, while the Curves command gives you more control. Use one or the other as needed to adjust your images, but you’ll never need both in a single project. In this article, we’ll look at the features of the Levels command and then look at how the Curves command expands upon its capabilities.

Photoshop levels curves lead

This article is compiled from Chapter 14 of Deke’s video course Photoshop CS5 One-on-One: Advanced. Like the last tip, it’s packed with useful information, but there’s even more value in viewing the actual chapter on lynda.com.

As Deke says at the beginning of the chapter, Levels and Curves are the left and right ventricles of Photoshop. The ventricles of the heart pump blood, and you could say that luminance levels (brightness) are the lifeblood of an image. Each command can be applied in two ways: A) as a one-shot, direct (static) adjustment to a pixel layer that can be applied through a dialog or a menu item, and B) as an adjustment layer that includes a mask, and which can be readjusted as much as you need.

The simplest way to work with either command is to apply it to a layer as an auto correction. However, applying at it as an adjustment layer is generally a better approach. The adjustment layer allows for more precise results and can be modified any time you need. We’ll begin with the direct versions of these commands and look more closely at the adjustment layer versions later in this article.

Auto Commands

To apply the adjustments as an auto-adjustment, duplicate your background layer and apply the auto command to the layer via the Image menu:

  • Select Auto Tone, Auto Contrast, or Auto Color from the Image menu -OR-
  • Select Image > Adjustments > Levels or Image > Adjustments > Curvesfrom the menu and then click the Auto button in the dialog. Clicking this button is equivalent to selectingImage > Auto Tone.
Photoshop auto adjustments

Auto Tone corrects luminosity on a channel-by-channel basis. The command affects the brightness and contrast of the image, as well as any color casts.

Auto Contrast corrects the composite image (i.e. it does not look at the image on a channel-by-channel basis). It works by looking at the distribution of luminance values in the image and moving the black and white points to the edges of that distribution. You should only use this command when the image does not have a color cast.

Auto Color examines the image channel-by-channel and corrects brightness/contrast, then seeks to neutralize highlights, shadows, and midtones in the image with the goal of getting rid of any color casts.

Auto Options

The Options feature provides six variations on the “flavor” of the Auto feature. In the Levels and Curves dialogs, you click the Options button to access the feature; in the adjustment layer version, you hold down the Alt key (Option on Mac) and click the Auto button.

open auto options

Even though the language is different, the three radio button options in this dialog correspond to Auto Tone, Auto Color, and Auto Contrast. Hover the cursor over the buttons to show a tooltip, explaining what each one does.

  • Enhance Monochromatic Contrast is the Auto Contrast command
  • Enhance Per Channel Contrast is Auto Tone
  • Find Light and Dark Colors corresponds to Auto Color.

The Snap Neutral Midtones check box creates two variations on each of the radio button options. When the Snap Neutral Midtones box is off, the command only adjusts the shadows and highlights in the image. Checking the box neutralizes the midtones as well. When it is switched on, it looks for a nearly-neutral color in the image and then adjusts the midtone values of the Red, Green, and Blue channels to “snap” the color to a neutral gray. When you select Image > Auto Color from the menu bar, it always applies the Snap Neutral Midtones option.

Manual Adjustments

When working with both Levels and Curves, the Black Point and White Point sliders determine which luminosity values will be forced to black and white respectively.

A single control called Gamma adjusts the distribution of middle tones in Levels. In Curves, the middle tones can be adjusted by adding one or more points to a graph that maps how tones will be redistributed.

Visualizing the Effects

Because Levels and Curves work by reassigning luminance levels, there’s always the possibility that tones will be clipped or dropped out in undesirable ways, so it’s a good practice to keep tabs on the Histogram panel as you use these commands. Except when using the dropper tools, the histogram inside Levels and Curves will display the pre-adjustment distribution of tones. Open the Histogram panel to see the post-adjustment distribution.

before and after histogram
  1. Use the Window > Histogram command to display the Histogram panel.
  2. Click the fly-out menu in the upper right corner of the panel to be sure that Expanded View is checked.
  3. Select RGB from the Channel menu to match the display’s appearance to that in the Levels and Curves displays.
  4. Be sure that the Source menu reads Entire Image.

One caveat: Photoshop uses cacheing to improve performance, so there may be times where the histogram will not be current. When that happens, Photoshop shows a triangular alert icon with an exclamation point inside the Histogram display. Click the icon to update the display.

show clipping option

The adjustment panels and dialogs for the Levels and Curves commands offer several ways to evaluate where clipping is occurring in the image as you adjust the Black and White points. One way is to select Show Clipping for Black/White Points from the fly-out menu at the upper right corner of the adjustments panel. With this setting, the screen will show the clipped areas in color as you move the sliders.

Gamma

The Gamma value in the Levels dialog adjusts midtones while leaving the black and white points fixed.

gamma slider

Drag the Gamma slider to the left to lighten, and to the right to darken. Be careful, over-brightening an image adds noise to shadows. For more precise control, you can click in the center box, set the value to 1 and then use the up or down arrow keys to nudge the value in increments of .1. You can also bump the value in larger increments of .1 by holding down the Shift key and tapping the up or down arrow keys.

Applying Levels or Curves as an Adjustment Layer


Whenever you add an adjustment layer, it’s a good idea to name it according to its function. You can add Levels or Curves adjustment layers three ways:

  1. Via one of the Layer > New Adjustment Layer > menu items
  2. By clicking the black and white circle icon at the bottom of the Layers panel (Create new fill or adjustment layer)
  3. By clicking the Levels or Curves icon in the first row of the Adjustments panel. If the Adjustments panel is showing adjustment controls instead of buttons, click the left-facing arrow at the bottom of the panel to get back to the buttons.
add adjustment layer

By the way, we say that adjustment layers are non-destructive, and that’s not really the case; they create the same (destructive) effect as static adjustments. It’s probably more accurate to call them reversible. You can readjust them as many times as you like or turn them off outright without having to start over, as you would with a static adjustment.

The Black point, White point, and Neutral Gray Droppers


Activate the Black point or White point dropper and click an area in the image to automatically move the Black point or White point slider to the corresponding tone in the graph. Clicking arbitrarily can easily cause the highlights or shadows to clip severely. If you click the wrong part of the image, simply click again to set a different value.

adjustment droppers

You can click an area of the image that’s supposed to be neutral gray with the Gray Balance dropper to adjust the color balance. See the video for details on how you can create a “gray card” by averaging the colors in the image itself, and then use the Gray Balance dropper to neutralize the image.

Noise can also make the colors of two adjacent pixels dramatically different. When that happens, the Point Sample setting for the dropper is likely to be too sensitive. The Sample Size setting of Eyedropper tool controls how all of the eyedroppers in Photoshop read the pixels within the image.Depending on the resolution of your image, using a setting of 3 x 3 or 5 x 5 average will generally work better. Tap the I key to activate the Eyedropper tool and use the Options bar to set the sample size.

eyedropper sample size

Up-Shifting to Curves

Curves picks up when Levels runs out of gas. For example, Levels is not very useful for reducing contrast, because it only works with the Black and White points. Curves also makes it possible to adjust contrast in just the highlights, midtones, or shadows, as needed. By comparison, all of the midtones are controlled by a single Gamma value in Levels, while Curves allows you to shape brightness and contrast by adding as many control points as you need.

Some features in Curves that are identical to those in Levels:

  • The eyedropper trio
  • The Auto button
  • The Options button
  • The Black point and White point droppers
curve and gamma

In Curves, placing a point at the center of the graph and moving it up or down is analogous to the gamma value, but it is expressed in input/output values. There is no way to map input/output to specific gamma values, but they have the same function. Drag straight up and down, or use the up or down arrow keys. Curves also has a pencil mode, where you can draw the shape of the curve arbitrarily. A smoothing option allows you to finesse the shape you’ve drawn by hand.

What’s totally different in curves:

curve dialog tone bubble
  • When you open the dialog (Image > Adjustment) version of curves, clicking or dragging in the image points to the corresponding tone in the graph by showing a bubble on the graph line.
  • Control-click (Command-click on Mac) to add a point to the graph. Then, you can use the arrow keys to move the point up or down.
  • When you have several points on the graph, you can select points with the keyboard. Use Control + Tab (both Windows and Mac) to cycle through the tabs, or tap the [+] or [-] keys.
  • Tap the Backspace (Delete on Mac) key to remove the selected point from the curve.

Curves as an Adjustment Layer

There are some slight differences in the interface of the adjustment layer version of Curves.

curves adjustment layer targeted
  • You can select the Eyedropper tool in the Tools panel (tap the I key) and Control-click (Mac: Command-click) in the image to add a point to the curve. If you drag the eyedropper before releasing the mouse button, a bubble will appear on the curve to indicate where the point will be placed.
  • The Targeted Adjustment tool greatly simplifies use of the Curves command, and is available in both versions of Curves. You’re likely to use it more than the eyedropper technique above. Click the icon to activate the tool, then drag up or down on parts of the image to select that tone in the graph and lighten or darken it.
  • The panel has a smoothing icon for the pencil mode, instead of a button.

Targeting Levels or Curves with Masking

You will undoubtably have cases where you want to open just some of the shadows, or to add more definition to specific highlights in an image. A targeted Levels or Curves adjustment is the perfect solution, allowing you to amp the contrast or shift the tones in just one part of the image.

If you make a selection before you add a Levels or Curves adjustment layer, the selection will be converted into a mask that applies the adjustment to the selected areas of the image. You can use any selection method you like, including the Color Range command or even the Quick Selection tool. These tools often make somewhat ragged selections, so once you’ve made your initial selection, you can click the Refine Edge button in the Options bar to clean up the transitions in the selection before you create the layer and its associated mask.

open shadows mask

Another masking technique is to use luminance mask, one of the simplest forms of a “found” mask. These are particularly useful for selectively applying a Curves adjustment layer.

  1. Open the channels panel, and look for the channel with the best contrast.
  2. Command-click on the channel to load it as a selection
  3. Click on the RGB channel to make the composite active
  4. Go to the Layers panel and add your Curves layer
load blue channel

Generally, the initial luminance mask may need some additional contrast or density in the blacks. The white areas may not be as clean as they could be either. You can apply a Levels adjustment to the mask to enhance the contrast.

  1. Alt-click (Option-click on Mac) on the mask to display it in the image editing area
  2. Select Image > Adjustments > Levels from the menu bar
  3. Adjust the black point and white point as needed and click OK
  4. Alt-click (Option-click on Mac) on the mask to show the image in the editing area
mask apply levels

The contrast adjustment can sometimes make some transitions in the mask too obvious. One way to resolve that is to click the mask to make sure that it is active (bracket marks will surround the edges of the mask thumbnail), and then use the Feather control in the Masks panel to soften the transition. Unlike the Levels adjustment, this setting can be revised or switched off, if you need to.

final configuration
barn before and after

Next entry:Deke’s Techniques 007: Blending Textures Onto a Face

Previous entry:dekeSpeak February 08, 2011

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