In addition to modifying or adjusting images with Selections , Filters , Layers , using Blend Mode s , and using Painting and Drawing  Tools, Photoshop  also allows you to adjust a wide variety of color properties, such as tone, saturation, balance, and contrast.  Controls for adjusting these image qualities are located under Image/Adjust.  Adjustment controls include Levels , Auto-Levels, Auto-Contrast , Curves , Color Balance , Brightness /Contrast, Hue /Saturation, Desaturate , Replace Color, Selective Color, Channel Mixer , Invert , Equalize, Threshold, Posterize, and Variations.  Before using these controls however, you should examine the image’s Histogram.

Histogram
The Histogram represents the pixels of an image in graph form.  It charts how many pixels an image contains and their distribution in terms of dark tones to bright tones.  Dark tones are charted to the left and bright to the right.  Dark tone areas are referred to as Shadows.  The area in between dark and bright tones is referred to as Midtones.  Bright tone areas are referred to as Highlights.  View the Histogram by selecting Image/Histogram.

Histogram for “priests.psd”

By examining the Histogram of an image, you can instantly see the Tonal balance of the image and use that information as a basis for your adjustments.

Channel
Select from Luminosity, Red, Green, or Blue when working with RGB images.  Luminosity is a composite of the Red, Green, and Blue Channels.

Mean, Std Dev, Median, Pixels
The values listed in the left hand column of the Histogram display the average brightness value of the image (Mean), the range of variance from the mean (Std Dev), the middle brightness value (Median), and the number of pixels used to calculate these numbers.  If you’re viewing a Histogram for the entire image, all the pixels are used.  If you’re viewing a Histogram for a selection, the numbers of pixels within the selection are used.

Level, Count, Percentile, Cache  Level
{Click} and drag to highlight particular areas of the Histogram to get values for Level (the selected area’s brightness level), Count (the number of pixels in the selected area), and Percentile (the percentage of pixels within the selected tonal range.)  Cache  Level is set in File/Preferences /Memory & Image Cache.  If Use Cache for Histograms  is checked, the Histogram is based on the Cache version of the image.  For absolute accuracy uncheck Use Cache for Histograms to force Photoshop  to use the real image data.  This is less of a concern for web development because you’re working on a low resolution (in terms of density as in 72 ppi) image, which matches the resolution of the Cache version.  However, if you are working on higher density images the accuracy of the Histogram is affected.

Using Levels
Open the buddha.psd image from the chapter006 folder and examine it visually.  You’ll notice that the image lacks contrast.  The whites aren’t particularly white and the blacks aren’t particularly black.  As a result, the statue doesn’t stand out from the background in terms of color brilliance, only in terms of shape and different, but subdued colors.  We can use Levels  to enhance the look of the statue and make it really stand out.  Select Image/Adjust/Levels.

Levels dialog box

TheLevels  dialog box contains a Histogram and controls for Channel, Input Levels, Input Level Sliders, Output Levels, Output Level Sliders, Shadows, Midtones, and Highlights Eyedroppers, a Preview option, and options to Load and Save Levels as well as designate Auto Levels .

Channel
Select from the composite RGB to modify all channels simultaneously or select Red, Green, or Blue and modify channels on an individual basis.

Input Levels /Input Levels Sliders
Input Levels  display the range from the Black Point (Shadows) to the White Point, and the Mid Point in between.  By default, the range is 256 possible values, from 0 to 255 with the Mid Point being precisely in between.  By adjusting these Levels, you can increase or decrease each of the three primary tonal properties, Shadows, Midtones, and Highlights.  The Histogram for the statue image reveals that very few pixels are actually pure white.  {Click} and drag the White Point Slider to the left until you reach a value of 220.  The white areas at the base of the statue become more brilliant.  The image is no longer muddy and the statue stands out from the background.  We could have done the same thing for the Shadows if the image had a deficiency in pure black pixels.

Output Levels /Output Levels Sliders
Reduce the tonal range (at default 0 to 255) by adjusting the Output Levels  either by input or using the sliders.

Levels  Eyedropper Tools
Use the Shadows, Midtones, and Highlights Eyedroppers to designate pure black, pure medium, and pure white manually by selecting the appropriate Eyedropper and {Clicking} on a point within the image.  If you select the Midtones Eyedropper and {Click} on the gray base of the lamp in the bottom left of the photograph the Histogram changes to reflect your selection and the color of the image changes as well.  In this case, the Blue Channel becomes dominant in the composite.  The Eyedroppers are generally more useful when changing the tone of grayscale images.

Preview
Check Preview to see the affect of your modifications in real-time.

Auto Levels
Selecting Image/Adjust/Auto Levels  causes Photoshop  to automatically adjust the tones in the image.

Auto Contrast
Select Auto Contrast  to automatically increase the contrast of colors in an image.  Colors increase in saturation and edges sharpen.

Curves
Use Curves  for greater control when adjusting image tone.  Curves are very similar to Levels  in that you adjust the Shadows (Black Point), Midtones (Medium Point), and Highlights (White Point), except you do so with a curve to which you can add points too for increased accuracy.  You manipulate the Curve by {Clicking} and dragging on a point.  You add points to the Curve by {Clicking} and dragging on the Curve where there isn’t a point or by {Single Clicking} on the Curve where there isn’t a point.  Controls for Curves include Channel, Input Values Bar and Field, Output Values Bar and Field, Edit Curve Button , Custom Curve Button, Black Point, Mid Point, and White Point Eyedroppers, Auto, Smooth, options to Save and Load Curve settings, the Curve Window and the Curve itself.  The Controls common to the Levels adjustment work precisely the same in Curves.

Curves  dialog box

Adjusting Tones with Curves
Open the buddha image again and select Image/Adjust/Curves .  The Curve is in a straight line, at 45 degrees, indicating it has not yet been manipulated.  The point at the bottom left of the Curve represents the current Black Point while the point at the top right represents the White Point.  There is no point in the middle of the Curve by default because you can add points all along the Curve and {Click} and drag them for very accurate and precise control.  However, by {Clicking} the center of the Curve you can add a Midtone Point.

{Click} the center of the Curve to add a Midtone Point.  Next {Click}, hold, and drag the Midtone Point up and down the Curve, trying to keep the Curve as straight as possible.  By doing this you are mirroring Levels  controls where you {Click} and drag the Point sliders back and forth.  The distance between the sliders or Points changes the tone of the image.  With Curves , you have increased control because you can add multiple points between the Black, Mid, and White Points.  {Click} twice at different locations on the Curve between the Midtone Point and the Black Point to add two more points.  Now {Click} and drag these points back and forth and watch what happens to the buddha image.  By {Clicking} and dragging the points to add variance to the Curve more drastic tonal changes occur.  It takes practice to get used to using Curves to adjust tone but as you can see it is a much more powerful option than using Levels.

Points Added to the Curve in the Curves  dialog box

Custom Curve Button
In addition to using the default Curve, Photoshop  permits you to draw your own Curve in the Curve Window.  Simply {Click} the Custom Curve Button  and {Click} and drag inside the Curve Window to create your Curve.  Adjust the Curve as you would normally by {Clicking} Edit Curve Button.

Color Balance
Adjust the Color Balance  of an image, Layer , or Selection with the Color Balance controls.  Controls include Color Balance fields and sliders for Cyan to Red, Magenta to Green, and Yellow to Blue, Tone Balance (Shadows, Midtones, and Highlights), Preserve Luminosity, and Preview.  Open the toys file from the chapter006 folder and convert it to RGB Mode  to explore Color Balance.

Color Balance  dialog box

Color Balance  Fields and Sliders
Use the Cyan to Red, Magenta to Green, and Yellow to Blue fields and sliders to adjust color values.  As you experiment with them, notice how the color changes in the image.  To isolate particular tones within the image use the Tone Balance controls.

Tone Balance
Check Shadows, Midtones, or Highlights to isolate Color Balance  changes you to those particular tones of the image.

Preserve Luminosity
Check Preserve Luminosity so that Color Balance  changes affect only the color of the pixels and not the brightness.

Brightness /Contrast
Use the Brightness  and Contrast  sliders to increase or decrease the brightness and contrast of an image, Layer , or selection.  Try increasing Contrast to maximum and see what happens to toys.

Brightness /Contrast  dialog box

Hue /Saturation
Use the Hue /Saturation Controls to increase and decrease the Hue, Saturation, and Lightness of an image, Layer , or selection.  From Edit select Master to adjust all the colors or select from a list of color ranges that include Reds, Yellows, Greens, Cyans, Blues, Magentas.  When you select a color range, you can adjust the range using the sliders in between the color bars at the bottom of the Hue/Saturation dialog box and by using the Eyedropper tools.  Experiment with toys.  {Clicking} Colorize causes the image, Layer, or selection to take on a duotone effect, where the image is tinted by one dominant color.

Hue Saturation dialog box

Desaturate
Selecting Desaturate  removes the color from an image.  The image will appear to be in Grayscale  Mode  but remains in RGB.

Replace Color
Select Replace Color to change the Hue , Saturation, and Lightness of a particular color or particular range of colors.  The Replace Color controls are similar to selecting by Color Range , including Fuzziness, Eyedroppers to select one color, select additional colors, or deselect colors, a Preview checkbox, and the ability to choose either Selection or Image in the Image Box.  In addition, the Replace Color controls include Hue, Saturation, and Brightness  sliders and fields, and the Sample Box that indicates the currently selected color or base color for the currently selected range.

Replace Color dialog box

Make sure you have toys open.  Select Image/Adjust/Replace Color.  Make sure Selection is selected for the Image Box and use the Eyedropper to {Click} the blue gown on one of the toys.  Adjust Fuzziness to 200 so that most of the content, other than the gowns, in the image are black.  You’ll notice that the blue area in the background of the photograph remains in the Image Box because the blue matches that of the gowns.  Now experiment with the Hue , Saturation, and Lightness controls to modify the color of the toys

Selective Color
Use Selective Color to adjust CMYK  values for images destined for print.  Because you don’t use CMYK mode for the Internet you won’t have much use for this control.  However, feel free to experiment with it.  Like other controls, you can isolate a color range and use sliders to modify them.

Channel Mixer
Use the Channel Mixer  to adjust the balance of each individual color channel in the image.  Select the desired Color Channel with the Output Channel drop-down list.  You’ll notice that sliders for the Source Channels default to a value of zero, except for the currently selected Color Channel, which defaults to a value of 100%.  Use the sliders to increase and decrease the percentage values of each Channel to change the tone of the image.  Use the Constant slider to add or subtract black or white to the mix.

Channel Mixerdialog box

Gradient Map
Use the Gradient Map dialog box to map either a custom or preset gradient to the grayscale values of an image.   The effect is somewhat like that of a Duotone  used in print but without the need of converting the image over to a Duotone. The default gradient consists of the current Foreground and Background Color , with the foreground mapped to black and the background mapped to white.  {Click} on the black triangle to the right of the Gradient Used for Grayscale  Mapping preview to select alternate gradients or {Double Click} on the gradient preview itself to edit and create custom gradients.   Editing Gradients  used as a Gradient Map is no different than editing those used as a fill so navigating your way through the options should pose no problem.  Check the Dither checkbox if you’re going to publish your graphic as an indexed image and need to reduce the banding of the gradient in your.   Check the Reverse checkbox to flip the colors from right to left.

Gradient Map dialog box

Three Color Gradient mapped to an image

Invert
Use Invert  to reverse the colors of the current image, Layer , or selection.  In effect, you make a photographic negative of your image.

Equalize
Use Equalize to make Photoshop  designate the lightest pixel and the darkest value as absolute black and white and then adjust the pixels between evenly or equally.

Threshold
Use Threshold on the current image, Layer , or selection to convert all pixels to either black or white.  Adjust the Threshold Level by enter a new value or using the slider.  Increasing the value adds more black and decreasing the value adds more white.  The range is from 0 to 255.

Posterize
Use Posterize on the current image, Layer , or selection to reduce the amount of colors, and therefore tone.  For instance, if you enter a value of 5, the image is reduced to five of the most dominant colors.

Variations
Use Variations to view multiple results of tonal changes for the current image, Layer , or selection.  The Variations dialog box consists of 12 Thumbnails  divided into three areas.  At the top is the image in its Original form beside which is the image as it appears with modifications.  To isolate tones and saturation control check Shadows, Midtones, Highlights, or Saturation.  Check Show Clipping when adjusting Saturation to ensure you do not over saturate a particular color.  Use the Fine and Coarse slider to adjust the amount of each change.

Affect changes by {Clicking} on the Thumbnails  in the Color or Brightness  areas.  The Color Area consists of Thumbnails to add more Green, Yellow, Cyan, Red, Blue, and Magenta.  Continue {Clicking} on the Thumbnails to add color.  Lighten or darken the image by {Clicking} on the Lighter or Darker Thumbnails in the Brightness area.  {Click} OK to affect changes.

Adjustment and Fill Layers
Photoshop  features a powerful Adjustment Layer  you can use within the Layers  Palette  to affect tonal changes to all the Layers that lie beneath the Adjustment Layer.  To create an Adjustment Layer select the Layer that you want to affect and {Click} on the New Fill  or Adjustment Layer button at the bottom of the Layers Palette.  A drop down menu will appear that allows you to choose a Layer Type.   For Fills select either Solid, Patter or Gradient and for Adjustments select either Levels , Curves , Brightness /Contrast , Color Balance , Hue /Saturation, Selective Color, Channel Mixer , Gradient Map, Invert , Threshold, or Posterize.

Adjustment Layer  Options

An Adjustment Layer  is created with the name of your selected Adjustment and the applicable dialog box appears.  Make changes as you would normally.  To modify your adjustments {Double-Click} on the Adjustment Layer.

Adjustment Layer Added