© Phillip Whitt 2020
P. WhittPractical Glimpsehttps://doi.org/10.1007/978-1-4842-6327-3_4

4. Correcting Exposure and Contrast

Phillip Whitt1 
(1)
Columbus, GA, USA
 

In this chapter, we’ll get to do some actual editing...finally! Glimpse is a great program for correcting problem areas such as exposure, contrast, and other tonal issues.

Note

The first few methods of making tonal corrections are fairly simple. They can be useful for making quick corrections (especially for beginners desiring to gain a little experience and confidence). The last part of the chapter introduces the most powerful tonal correction tools (Levels and Curves)—they take more time to master, but it’s time well invested.

The topics covered in this chapter are

  • Tutorial 1: Using the Exposure dialog

  • Tutorial 2: Lightening dark areas using the Shadows-Highlights dialog

  • Tutorial 3: Improving contrast using the Brightness-Contrast dialog

  • The Levels dialog

  • Understanding the histogram

  • Tutorial 4: Improving contrast using Levels

  • The Curves dialog

  • Tutorial 5: Improving local tonality using Curves

  • Chapter conclusion

Note

Most digital photos (particularly scanned film-based photographs) can benefit from corrective sharpening. In most of the tutorials throughout this book, we’ll use the Unsharp Mask filter as the last step. Basically, this filter improves the apparent sharpness of the image by increasing contrast in the edges of the elements in the image. For a detailed description of the Unsharp Mask filter, click here: https://docs.gimp.org/2.6/en/plug-in-unsharp-mask.html.

Tutorial 1: Using the Exposure Dialog

In this lesson, the Exposure dialog will be used to brighten an old photograph that was underexposed when it was shot (which means the light levels were too low when the film was exposed). Using the Exposure dialog mimics increased light levels by increasing the brightness of every pixel in the image.

  1. 1.

    Open the practice image Ch_4_Dark_Boat in Glimpse.

     
  2. 2.

    Create a duplicate of the background layer (Right-Click ➤ Duplicate Layer) and rename it (Work Layer, or something similar).

     
  3. 3.

    Launch the Exposure dialog (Colors ➤ Exposure)—by ticking the Split view option, a side-by-side view comparing the effect applied shown against the original is displayed.

     
  4. 4.

    Set the Exposure value to about 2.25 (Figure 4-1)—do not click the OK button just yet.

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig1_HTML.jpg
Figure 4-1

Setting the Exposure value with the Split view option enabled

  1. 5.

    This action caused the areas that are supposed to be dark to become muddy, so now we’ll offset this by adjusting the Black level. Set the Black level value to 0.002, then click OK (Figure 4-2).

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig2_HTML.jpg
Figure 4-2

Setting the Black level value darkens the shadow areas that became muddy

  1. 6.

    The last thing to do is to slightly sharpen the image. Open the Unsharp Mask dialog (Filters ➤ Enhance ➤ Sharpen (Unsharp Mask)).

     
  2. 7.

    Set the Radius to 1.00, leaving Amount and Threshold at their default settings (Figure 4-3), then click OK.

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig3_HTML.jpg
Figure 4-3

Setting the Radius in the Unsharp Mask dialog

Figure 4-4 shows the before and after comparison . When done, you can save as an .XCF file if you want to keep the file, or simply close it out.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig4_HTML.png
Figure 4-4

The before and after comparison

Overview of the Shadows-Highlights Dialog

Shadows-Highlights is a bit more complex than most of the other dialogs in Glimpse. It’s used to control exposure in shadow and highlight areas in an image, allowing for precise control (Figure 4-5).
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig5_HTML.jpg
Figure 4-5

The Shadows-Highlights dialog

The Shadows-Highlights dialog’s functions are as follows:
  1. 1.

    Presets—Allows the user to save the current value settings; this is useful when reapplying the same adjustments to multiple images, eliminating the need to manually adjust the settings each time.

     
  2. 2.

    Shadows—Adjusts the exposure in the shadow (darkest) areas of the image.

     
  3. 3.

    Shadows Color Adjustment—Adjusts the color saturation in the shadow (darkest) areas of the image.

     
  4. 4.

    Highlights—Adjusts the exposure in the highlight (lightest) areas of the image.

     
  5. 5.

    Highlights Color Adjustment—Adjusts the color saturation in the highlight (lightest) areas of the image.

     
  6. 6.

    White Point Adjustment—Shifts the white point of the image.

     
  7. 7.

    Radius—Spatial extent (essentially expands or narrows range of effect). Figure 4-6 shows an example of the shadows being lightened, and the radius expanding the range a bit further (note the shadows cast on the ground).

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig6_HTML.png
Figure 4-6

A comparison of the original image, the Shadow slider, and the Radius slider

  1. 8.

    Compress—Compresses the effect while preserving midtones. Normally, the subject in the photograph is in the midtone range and has been adjusted for in the camera’s settings, so it’s usually desirable to preserve these.

     
  2. 9.

    Split View—Provides a side-by-side view of the image with and without effect applied.

     

Tutorial 2: Lightening Dark Areas Using the Shadows-Highlights Dialog

In this lesson, the Shadows-Highlights dialog will be used to lighten a portion of a digital photograph that is underexposed while preserving the midtones and highlights.

  1. 1.

    Open the practice image Ch4_Dark Foreground in Glimpse.

     
  2. 2.

    Create a duplicate of the background layer (Right-Click ➤ Duplicate Layer) and rename it (Work Layer, or something similar).

     
  3. 3.

    Launch the Shadows-Highlights dialog (Colors ➤ Shadows-Highlights)—by ticking the Split view option, a side-by-side view comparing the effect applied shown against the original is displayed.

     
  4. 4.

    Set the Shadows slider to 60.00-65.00, the White point adjustment to 2.00, and the Radius slider to 10.00; click OK when done (Figure 4-7).

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig7_HTML.jpg
Figure 4-7

Set the Shadows slider to 60.00–65.00, the White point adjustment to 2.00, and the Radius slider to 10.00, then click OK

  1. 5.

    The last thing to do is to slightly sharpen the image. Open the Unsharp Mask dialog (Filters ➤ Enhance ➤ Sharpen (Unsharp Mask)).

     
  2. 6.

    Set the Radius to 1.00, leaving Amount and Threshold at their default settings (Figure 4-8), then click OK.

    Note I zoomed in a great deal to show the subtle but noticeable difference that the Unsharp Mask filter made.

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig8_HTML.jpg
Figure 4-8

Setting the Radius in the Unsharp Mask dialog

Figure 4-9 shows the before and after comparison. When done, you can save as an .XCF file if you want to keep the file, or simply close it out.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig9_HTML.png
Figure 4-9

The before and after comparison

Tutorial 3: Improving Contrast Using the Brightness-Contrast Dialog

In this tutorial, we’ll call on the easy-to-use Brightness-Contrast dialog. It can be a useful dialog, but it’s a bit limited compared to other tonal adjustment dialogs (such as Levels or Curves).

It’s a useful feature for making minor global (overall) tonal corrections in dull images. But more often than not, it lacks the precision to make a correction without overdoing the result. Figure 4-10 shows how easy it is to blow out the highlights and lose detail.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig10_HTML.jpg
Figure 4-10

The Brightness-Contrast dialog can “overdo” the result, such as blowing out highlights

Now we’ll use the Brightness-Contrast dialog to improve an old photo that has dull contrast.

  1. 1.

    Open the practice image Ch4_Baby_in_Bath in Glimpse.

     
  2. 2.

    Create a duplicate of the background layer (Right-Click ➤ Duplicate Layer) and rename it (Work Layer, or something similar).

     
  3. 3.

    Open the Brightness-Contrast dialog (Colors ➤ Brightness-Contrast).

     
  4. 4.

    Set the Contrast slider to 53; click OK when done (Figure 4-11).

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig11_HTML.jpg
Figure 4-11

Set the Contrast slider to 53 to boost the contrast in this dull image

  1. 5.

    Open the Unsharp Mask dialog (Filters ➤ Enhance ➤ Sharpen (Unsharp Mask)).

     
  2. 6.

    Because this is an old image, we’ll sharpen it a bit more than the previous ones. Leave at the default settings (Radius 3.0, Amount 0.50, and Threshold 0.0), then click OK (Figure 4-12).

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig12_HTML.jpg
Figure 4-12

Leave the Unsharp Mask dialog at the default settings

Figure 4-13 shows the before and after comparison. When done, you can save as an .XCF file if you want to keep the file, or simply close it out .
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig13_HTML.jpg
Figure 4-13

The before and after comparison

The Levels Dialog

The Levels dialog is an extremely useful feature. Levels can be used to make tonal corrections, such as making an image lighter, darker, or boosting the contrast with great precision.

Figure 4-14 shows the Levels dialog, with a brief description on most of the functions.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig14_HTML.jpg
Figure 4-14

The Levels dialog

The Levels dialog’s functions are as follows:

  1. 1.

    Presets—Allows the user to save the current value settings; this is useful when reapplying the same adjustments to multiple images, eliminating the need to manually adjust the settings each time.

     
  2. 2.

    Channel—Adjusts the composite channel, or the red, green, and blue channels individually.

     
  3. 3.

    Input Levels—The tonal information is essentially “remapped” by using the black, midtone, or white sliders. The numeric input boxes can also be used. The eye drop icons are used to pick black or white points for the selected channel (composite or the red, green, or blue channel).

     
  4. 4.

    Output Levels—Allows the output level range to be limited or constrained; an example would be moving the white slider to the left set a lower maximum value for the highlights.

     
  5. 5.

    Auto Input Levels—Automatically adjusts levels for all channels.

     
  6. 6.

    Preview—When this box is checked, the effect is shown in real time as the adjustments are being made.

     
  7. 7.

    Adjust Levels Perceptually/Linear Histogram—These are the default levels adjustment and histogram readout settings.

     
  8. 8.

    Pick Black, Gray, or White Points for All Channels—Makes tonal adjustments in the composite channel.

     
  9. 9.

    Edit These Settings As Curves—Instantly switches from Levels to the Curves dialog.

     
  10. 10.

    Split View—A side-by-side view comparing the effect applied shown against the original is displayed.

     

Understanding the Histogram

The Levels dialog displays a histogram , which is a graphical representation of the pixel brightness values ranging from 0 (100% black) to 255 (100% white).

Figure 4-15 shows a black-and-white (grayscale, to be technically correct) image with the Levels dialog open. The tonal information of the image of the car is mapped in the histogram. The darkest pixels are in the left portion of the histogram, the middle-range pixels in the center portion, and the lightest pixels in the right portion.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig15_HTML.jpg
Figure 4-15

The tonal information of this image is mapped in the histogram

If you’ll notice, most of the data spans most of the length of the histogram with no large gaps.

However, in Figure 4-16, we can see there is a gap in the shadows portion, and an even larger gap in the highlights portion. The image is dull, a bit dark, and lacking contrast—the lack of highlight values is the main reason.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig16_HTML.jpg
Figure 4-16

There are gaps in the shadow and highlight portions of the histogram

Tutorial 4: Improving Contrast Using Levels

Now we’ll use the Levels dialog to improve the contrast of the same image that the Brightness-Contrast dialog was used on.
  1. 1.

    Open the practice image Ch4_Baby_in_Bath in Glimpse.

     
  2. 2.

    Create a duplicate of the background layer (Right-Click ➤ Duplicate Layer) and rename it (Work Layer, or something similar).

     
  3. 3.

    Open the Levels dialog (Colors ➤ Levels).

     
  4. 4.
    Now, we’ll make the following adjustments (Figure 4-17):

    • Move the black slider (input levels) to the right until the value is 76.

    • Move the white slider (input levels) to the left until the value is 195.

    • Click OK when done.

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig17_HTML.jpg
Figure 4-17

Move the black and white sliders as shown to bring out contrast

  1. 5.

    As we did in the previous tutorial, we’ll sharpen the image a bit. Launch the Unsharp Mask filter dialog (Filters ➤ Enhance ➤ Sharpen (Unsharp Mask)).

     
  2. 6.

    Leave at the default settings (Radius 3.0, Amount 0.50, and Threshold 0.0), then click OK.

     
Figure 4-18 shows the before and after comparison. When done, you can save as an .XCF file if you want to keep the file, or simply close it out.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig18_HTML.png
Figure 4-18

The before and after comparison

The Curves Dialog

The Curves dialog is another useful feature—like levels, curves can be used to make tonal corrections and color corrections, but it offers greater precision.

Figure 4-19 shows the Curves dialog, with a brief description on most of the key functions.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig19_HTML.jpg
Figure 4-19

The Curves dialog

The Curves dialog’s functions are as follows:

  1. 1.

    Presets—Allows the user to save the current value settings; this is useful when reapplying the same adjustments to multiple images, eliminating the need to manually adjust the settings each time.

     
  2. 2.

    Channel—Adjusts the composite channel, or the red, green, and blue channels individually.

     
  3. 3.

    Curve—This line can be curved to make adjustments in tone or color.

     
  4. 4.

    Histogram—Graphical display of the pixel brightness.

     
  5. 5.

    Input/Output—Displays the x/y position of the cursor on the grid as the curve is being adjusted.

     
  6. 6.

    Preview—When this box is checked, the effect is shown in real time as the adjustments are being made.

     
  7. 7.

    Adjust Curves Perceptually/Linear Histogram—Default settings; the other two settings are Adjust curves in linear light and Logarithmic histogram.

     
  8. 8.

    Type—Configure as a curve or angled; the Curve type option offers smooth or freehand; an option of a curve with rounded arcs, or a pencil-like tool that allows the user to draw sharp angles on the curve (Figure 4-20).

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig20_HTML.jpg
Figure 4-20

The Smooth and Freehand curve types

  1. 9.

    Split View—A side-by-side view comparing the effect applied shown against the original is displayed.

     
The example in Figure 4-21 shows a comparison of the Smooth and Corner settings. Depending on the adjustment being made, one setting may be better suited than the other. For example, the Smooth setting produces a more gradual effect (and, in most cases, is probably the most useful setting), and the Corner setting produces slightly different tonal changes.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig21_HTML.jpg
Figure 4-21

A comparison of the Smooth and Corner curve settings

The example in Figure 4-22 shows an example of an “S” curve. The center of the line is anchored while a slight s-shape is made to improve dull contrast. The center of the line represents the areas in the image that are mainly average in luma and tone; an anchor point is placed by clicking the line.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig22_HTML.jpg
Figure 4-22

An example of an “S” curve, which is used to improve contrast

Tutorial 5: Improving Local Tonality Using Curves

Now we’ll use the Curves dialog to reduce the harshness in the shadows of a slightly underexposed image. This adjustment localizes the darkest areas and lightens them slightly while leaving the midtones and highlights mostly unaffected.

  1. 1.

    Open the practice image Ch4_Lighten_Shadows in Glimpse.

     
  2. 2.

    Create a duplicate of the background layer (Right-Click ➤ Duplicate Layer) and rename it (Work Layer, or something similar).

     
  3. 3.

    Open the Curves dialog (Colors ➤ Curves).

     
  4. 4.

    We’ll make a slight bend in the curve to lighten the shadows. As shown in Figure 4-23, place an anchor point on the grid (input: 160/output: 160), and bend the curve slightly upward (input: 64/output 160).

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig23_HTML.jpg
Figure 4-23

Adjust the curve as shown to lighten the shadows

  1. 5.

    Now, we’ll sharpen the image a bit. Launch the Unsharp Mask filter dialog (Filters ➤ Enhance ➤ Sharpen (Unsharp Mask)).

     
  2. 6.

    Set the Radius to 1.00, leaving Amount and Threshold at their default settings (Figure 4-24), then click OK.

     
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig24_HTML.jpg
Figure 4-24

Setting the Radius in the Unsharp Mask dialog

Figure 4-25 shows the before and after comparison. The effect is just enough to lighten the shadows a little. When done, you can save as an .XCF file if you want to keep the file, or simply close it out.
../images/495886_1_En_4_Chapter/495886_1_En_4_Fig25_HTML.png
Figure 4-25

The before and after comparison

Chapter Conclusion

This chapter provided several tutorials on correcting tonal problems (such as exposure and contrast).

Here are the main topics that were covered:

  • Using the Exposure dialog

  • Lightening dark areas using the Shadows-Highlights dialog

  • Improving contrast using the Brightness-Contrast dialog

  • The Levels dialog

  • Understanding the histogram

  • Improving contrast using Levels

  • The Curves dialog

  • Improving local tonality using Curves

In the next chapter, we’ll look at working with color and making color corrections.

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