The Photoshop CS5 Pocket Guide: Making Corrections
Unless you're working with an adjustment layer, most image corrections involve changing the actual pixels. So, once again, it's a good idea to be working in a copy of the image, leaving your original intact for future use.
The tonality of an image includes qualities of lightness, darkness, and contrast. Photoshop includes a full range of tools to correct or enhance the tonality of an image. Some of the tools are fairly straightforward, while others are a bit more complex. They all work essentially the same way, however: they map the original pixel values to a new range of values.
If you're working on an image that you expect to print professionally or for some other reason the image tone is absolutely critical, make sure you've calibrated your monitor and you're using the appropriate color management profile. (For help calibrating your monitor, see Photoshop Help.) Additionally, make changes in the color mode you'll be using for the final output. If the image is intended for the Web, work in RGB mode. If you plan to print the image professionally, either work in CMYK mode or convert to CMYK just before you print. But don't move back and forth between the modes if you can avoid it.
Generally, the following workflow will get you to good results most efficiently:
- Remove flaws such as dust spots or scratches before making color and tonal adjustments.
- View the histogram for the image to see its quality and tonal range.
- Start by adjusting the color balance to remove any color casts or correct saturation.
- Adjust the tonal range using either a Levels or Curves adjustment layer. There, you'll set the white point (defining highlights) and black point (defining shadows), and make any necessary adjustments to the midtones.
- Finally, make any other color adjustments, such as hue, saturation, or vibrance.
To open the Histogram panel, choose Window > Histogram. The Levels dialog box also displays a histogram.
A histogram shows you how pixels are distributed in an image, so you can see how much detail appears in the shadows (on the left), the highlights (on the right), and the midtones (in the middle). Much of the time, you'll want an even distribution—an image that is neither too dark nor too light. However, in some cases, it's appropriate for the detail to be concentrated in the shadows or the highlights, depending on the mood you want to convey.
Figure 4.1 The histogram shows how pixels are distributed. In the first image, there's more detail in the highlights and midtones. The second image has more detail in the shadows, with some in the highlights.
You can perform a generalized color correction using a Color Balance adjustment layer, or you can let Photoshop take a stab at it by using the Auto Color option in the Levels or Curves dialog box. Balancing color removes any color cast, such as a blue or red cast, that may appear in your image.
To add a Color Balance adjustment layer:
- Click the Color Balance icon in the Adjustments panel.
- Select Shadows, Midtones, or Highlights as an area where you want to focus the changes.
- Drag the sliders toward the colors you want to increase and away from the colors you want to decrease. For example, if the image has a red cast, drag the slider away from Red and toward Cyan. Be careful not to overcorrect—you can easily add a color cast using the Color Balance adjustment!
To remove a color cast using the Auto Color option:
- Click the Levels or Curves icon in the Adjustments panel.
- Click Auto.
The Auto option neutralizes midtones and clips shadows and highlight pixels.
Figure 4.2 Use a Color Balance adjustment layer to remove color casts or otherwise adjust color.
The Levels panel affects the levels of intensity of shadows, midtones, and highlights. It's not all that hard to make the adjustments. The histogram helps you see where the detail is in the image, and there are some rules of thumb for improving it.
In the Levels histogram, the left side represents the shadows, the right side represents the highlights, and the center shows the midtones. The input and output sliders are related, but here's what you need to know to use this adjustment: The left input slider, at the bottom of the histogram, sets the image's black point—the point beyond which all data is seen as pure black. The right input slider sets the white point—the point beyond which everything is treated as pure white. Generally, you'll get better results if you move the left slider to the point where significant shadow data occurs, and the right slider to the point where significant highlight data begins.
The middle slider adjusts the gamma, which changes the intensity of the middle range of gray tones without making large changes to highlights and shadows.
You'll hear photo-editing pros talk about clipping shadows or highlights, something you typically want to avoid. When shadows or highlights are clipped, the pixels are a solid white or black and provide no detail.
Figure 4.3 Adjusting levels can be a quick way to punch up the intensity of the color in your image, and to ensure you're seeing the appropriate detail.
To apply a Levels adjustment:
- Click the Levels icon in the Adjustments panel.
- Drag the black and white input levels sliders to the edge of the first group of pixels at either end of the histogram. Photoshop sets the black and white points at the new position—everything to the left of the black point is solid black; everything to the right of the white point is solid white.
- Adjust the middle slider to tweak the midtones. Moving it to the left lightens the image; moving it to the right darkens the image.
To neutralize a color cast using the Levels adjustment:
- Click the Levels icon in the Adjustments panel.
- Click the Set Gray Point Eyedropper tool.
- Click in a part of the image that should be neutral gray.
The Curves adjustment has nothing to do with the bodaciousness of your subject. You use the Curves adjustment to make more gradual changes throughout the tonal range than you can through the Levels adjustment (which has only three points of adjustment: black, white, and gamma). Why curves? Because the adjustments appear on a graph, and as you adjust the tonal range, you're adjusting the curve of the line.
The horizontal axis of the graph represents the input values; the vertical axis represents the output values. Initially, the tonal range is represented as a straight diagonal baseline because the input and output values are identical. By default, the Curves graph shows the amount of light, so moving the curve upward lightens the image and moving it downward darkens the image. The steeper sections of the curve represent areas of higher contrast, and flatter sections represent lower contrast. Moving a point in the top portion of the curve adjusts the highlights; moving a point in the center adjusts the midtones; and moving a point in the bottom section adjusts the shadows. You generally need to make only small curve adjustments to correct the tone and color in an image.
Figure 4.4 Adjust the curves to make more gradual changes through the tonal range.
To adjust curves:
- Click the Curves icon in the Adjustments panel.
- Add a point on the curve by clicking directly on it. You can add up to 14 points to a curve.
- Use the black point and white point sliders to set the black and white points. Typically, you want the black point to be at the point that the shadow data appears, and the white point to be at the point that the highlight data appears.
- Click a point, and drag the curve until you're satisfied with the tone and color of the image. Or, enter new values for the point in the Output and Input boxes.
You can also select the On-image adjustment tool, and then click the area in the image you want to adjust. Drag the point up or down to lighten or darken the values for all similar tones in the image.
The Vibrance adjustment saturates the colors that need it while leaving the colors that are already saturated alone. It's particularly useful if you're working with an image that contains skin tones, as it keeps them from becoming oversaturated.
- Click the Vibrance icon in the Adjustments panel.
- Drag the Vibrance slider to increase or decrease color saturation.
- To apply the same amount of saturation adjustment to all colors (even those that are already saturated), move the Saturation slider.
Figure 4.5 Adjust the Vibrance slider to intensify some colors without oversaturating others.
Correcting shadows and highlights
If an image was taken with strong backlighting, resulting in silhouettes—or if the subject of the image was too close to the flash and ended up kind of washed out, the Shadows/Highlights command may be your best bet for correction. The Shadows/Highlights command doesn't just lighten or darken an image; instead, it lightens or darkens each pixel based on nearby pixels in shadows or highlights. You can make adjustments to shadows and highlights separately. The defaults are set to fix images with backlighting problems.
To adjust image shadows and highlights:
- Choose Image > Adjustments > Shadows/Highlights.
- Move the Amount slider or enter a percentage in the Shadows or Highlights box. Larger values lighten shadows more or darken highlights more.
- Click OK when you're happy with the adjustment, or select Show More Options and make fine-tuned adjustments.
Selecting a variation
If you're overwhelmed by tone correction options, or if you're not sure what might make a particular image look better, try the Variations command. The Variations dialog box shows you alternatives that present different color balance, contrast, and saturation settings, so you can select the one that achieves the look you want.
Figure 4.6 Correct lighting issues using the Shadows/Highlights adjustment; select Show More Options to tweak the settings further.
Choose Image > Adjustments > Variations.
The Variations dialog box shows two thumbnails at the top of the dialog box: the original settings and the current selection. The current selection thumbnail changes as you make choices.
- Select what to adjust in the image. You can adjust shadows, midtones, or highlights, as well as saturation.
- Drag the Fine/Coarse slider to change the amount of each adjustment. (Each tick doubles the adjustment amount.)
- Adjust the color and brightness:
- To add a color, click the appropriate thumbnail.
- To subtract a color, click the thumbnail for its opposite color.
- To adjust brightness, click a thumbnail on the right side of the dialog box.
Figure 4.7 Visual types, take heart. You can see the effects of different color and contrast options in the Variations dialog box.