Photoshop can provide magical transformations to images, making them unrecognizable from the original, but sometimes what you really want is simply to make an image look the same as the original — only better. Perhaps the colors are a little too warm, or the shadows a bit inky. Whatever’s wrong with the image, the last thing you want is to change it so much that it looks like it’s been processed more than a freeze-dried floral arrangement. You’d be happy to have everyone admire your image without a clue that you’d made major corrections in Photoshop.
Welcome to the world of image enhancements. This section concentrates on the things you can do to correct color, contrast, hue, and color saturation. After you master the basic tools, you’ll want to explore some even more sophisticated things you can do by using features like Photoshop’s Adjustment Layers which let you dynamically apply your changes in remarkably flexible ways. But before you dive into image adjustments, you’ll want to make sure and remove any flaws, such as dust, scratches, blemishes, and other nasty items, from your image.
Introducing the Histogram Palette
One of the first things you want to do before you make any color or tonal adjustments to your image is to take a good look at the quality and distribution of the tones throughout your image. I don’t mean just eyeballing the composite image on your screen. I’m talking about getting inside your image and looking at its guts. This just got easier with the introduction of the Histogram palette. Sure. You could access the histogram of a image through dialog boxes such as the Levels command, but now you’ve been given the histogram in a palette that can stay on-screen, giving you constant feedback on your image adjustments.
A histogram displays the tonal range (also referred to as the key type) of an image. It shows how the pixels are distributed by graphing the number of pixels at each of the 256 brightness levels in an image. On this graph, pixels with the same brightness level are stacked in bars along a vertical axis. The higher the line from this axis, the greater the number of pixels at that brightness level. You can view the distribution for each color channel separately or for the composite image as a whole. From this graph, you can then determine whether the image contains enough detail in the shadow, midtone, and highlight areas. This information helps you determine what image adjustments you may need to make. For example, an image with good tonal range displays pixels in all areas. An image with poor tonal range has gaps in the histogram.
Using the Histogram palette
Choose Window-->Histogram to bring up this graphical wonder. By default, the histogram displays the tonal range of the whole image. However, you can also make a selection to view the tones of just that portion. Or you can even select a single channel to view the tones of that individual channel. When you first bring up the Histogram palette, it opens in Compact View by default. No controls or statistics are visible. To access the various views, go to the Histogram palette pop-up menu. Here is a description of each view:
- Compact View: Displays only a histogram of the whole image (or your chosen selection or channel) with no controls or statistics.
- Expanded View: Shows a histogram with statistics and controls for choosing and viewing the histogram of individual channels. There are also controls for refreshing the histogram to show uncached data, and choosing a selected layer (on the Source pop-up menu).
- All Channels View: Displays all the options of the Expanded View, plus shows the individual histograms for each color channel. Note that the individual histograms don’t include alpha channels, spot channels, or masks.
Here is a laundry list of other facts about using the Histogram palette:
- The default display option of the histogram is the composite image’s color mode, such as RGB, CMYK, Grayscale, and so on.
- If you are in Expanded or All Channels View, you can choose to display the histogram of an individual channel. The palette also displays alpha channels and spot channel.
- Other options on the Channel pop-up menu include Luminosity and Colors. Luminosity displays a histogram which shows the luminance (or lightness component) of the image. Colors displays a composite histogram of each color channel in color. Note that Photoshop retains and displays the option you choose on the Channel pop-up menu — even if you switch views.
- To view your channels in color, choose Show Channels in Color from the palette pop-up menu. If you want to see the channel in color in Compact View, first choose the channel in Expanded or All Channels View, choose Show Channels in Color, and then switch to Compact View.
- To hide the statistics in the Expanded and All Channels views, deselect Show Statistics from the Histogram palette pop-up menu.
- Position your cursor within the histogram to see statistics about a specific value. Drag your cursor within the histogram to see statistics about a range of values (Photoshop highlights the range).
- By default the histogram displays the values of your entire image, which includes all layers.
- To display the histogram of a single layer, choose Selected Layer from the Source popup menu. Photoshop displays the histogram of the layer that’s currently selected in the Layers palette. If the image doesn’t have layers, the option isn’t available.
- To show the histogram of an adjustment layer, select Adjustment Composite from the Source pop-up menu. Photoshop displays the histogram of the adjustment layer that’s currently selected in the Layers palette (and all the layers below that adjustment layer).
- Be sure to select any Preview options in the dialog boxes of your image adjustments, such as Levels. That way, the Histogram palette displays both the original and adjusted histograms.
- The Cached Data Warning icon lets you know that Photoshop is reading the histogram from cache instead of your image’s current state. Cache is a reserved, high-speed section of your computer’s memory. The image cache allows the histogram to display faster because it is calculating the histogram based on a representative sampling of the pixels in your image. If you want to refresh the histogram to display all the pixels in your image in its current state, click the Uncached Refresh button. You can also choose this command from the palette pop-up menu. Or simply double-click anywhere in the histogram. Unless it is really bogging down your workflow, I recommend viewing your image’s histograms using uncached data.
Select the Use Cache for Histograms option in the Memory and Image Cache panel of your Preferences if you wish to utilize cached image data in your histogram.
Understanding the histogram statistics
The Histogram palette gives you all kinds of statistics about the pixels in your image. Some of these statistics, such as Standard Deviation, may be for those who live in the land of Calculus. But you may be able to glean some useful information from some of the other statistics that can help you in your image-adjusting tasks. Here’s a brief explanation of each statistic:
- Mean: Average intensity value
- Standard Deviation: How much the intensity values vary
- Median: Middle value of the intensity value range
- Pixels: Total number of pixels used to represent the histogram
- Cache Level: The current level of image cache used to calculate the histogram
As mentioned in the preceding section, when the histogram is calculated using image cache it is basing it on a sampling of the pixels in your image. The original image is cache level 1. For every level above that, Photoshop averages four adjacent pixels below to get one pixel, so each level is half the dimensions (1⁄4 the number of pixels) of the level below. When you choose the Uncached Refresh command, the cache level reverts to 1.
The next three statistics display a value only when you position or drag your cursor in the histogram. Each value corresponds only to the portion of the histogram under your cursor.
- Level: Intensity level
- Count: The total number of pixels corresponding to that intensity level
- Percentile: The number of cumulative pixels (in percentages) at or below that level, from 0% (left) to 100% (right)
Welcome to the world of image enhancements. This section concentrates on the things you can do to correct color, contrast, hue, and color saturation. After you master the basic tools, you’ll want to explore some even more sophisticated things you can do by using features like Photoshop’s Adjustment Layers which let you dynamically apply your changes in remarkably flexible ways. But before you dive into image adjustments, you’ll want to make sure and remove any flaws, such as dust, scratches, blemishes, and other nasty items, from your image.
Introducing the Histogram Palette
One of the first things you want to do before you make any color or tonal adjustments to your image is to take a good look at the quality and distribution of the tones throughout your image. I don’t mean just eyeballing the composite image on your screen. I’m talking about getting inside your image and looking at its guts. This just got easier with the introduction of the Histogram palette. Sure. You could access the histogram of a image through dialog boxes such as the Levels command, but now you’ve been given the histogram in a palette that can stay on-screen, giving you constant feedback on your image adjustments.
A histogram displays the tonal range (also referred to as the key type) of an image. It shows how the pixels are distributed by graphing the number of pixels at each of the 256 brightness levels in an image. On this graph, pixels with the same brightness level are stacked in bars along a vertical axis. The higher the line from this axis, the greater the number of pixels at that brightness level. You can view the distribution for each color channel separately or for the composite image as a whole. From this graph, you can then determine whether the image contains enough detail in the shadow, midtone, and highlight areas. This information helps you determine what image adjustments you may need to make. For example, an image with good tonal range displays pixels in all areas. An image with poor tonal range has gaps in the histogram.
Using the Histogram palette
Choose Window-->Histogram to bring up this graphical wonder. By default, the histogram displays the tonal range of the whole image. However, you can also make a selection to view the tones of just that portion. Or you can even select a single channel to view the tones of that individual channel. When you first bring up the Histogram palette, it opens in Compact View by default. No controls or statistics are visible. To access the various views, go to the Histogram palette pop-up menu. Here is a description of each view:
- Compact View: Displays only a histogram of the whole image (or your chosen selection or channel) with no controls or statistics.
- Expanded View: Shows a histogram with statistics and controls for choosing and viewing the histogram of individual channels. There are also controls for refreshing the histogram to show uncached data, and choosing a selected layer (on the Source pop-up menu).
- All Channels View: Displays all the options of the Expanded View, plus shows the individual histograms for each color channel. Note that the individual histograms don’t include alpha channels, spot channels, or masks.
Here is a laundry list of other facts about using the Histogram palette:
- The default display option of the histogram is the composite image’s color mode, such as RGB, CMYK, Grayscale, and so on.
- If you are in Expanded or All Channels View, you can choose to display the histogram of an individual channel. The palette also displays alpha channels and spot channel.
- Other options on the Channel pop-up menu include Luminosity and Colors. Luminosity displays a histogram which shows the luminance (or lightness component) of the image. Colors displays a composite histogram of each color channel in color. Note that Photoshop retains and displays the option you choose on the Channel pop-up menu — even if you switch views.
- To view your channels in color, choose Show Channels in Color from the palette pop-up menu. If you want to see the channel in color in Compact View, first choose the channel in Expanded or All Channels View, choose Show Channels in Color, and then switch to Compact View.
- To hide the statistics in the Expanded and All Channels views, deselect Show Statistics from the Histogram palette pop-up menu.
- Position your cursor within the histogram to see statistics about a specific value. Drag your cursor within the histogram to see statistics about a range of values (Photoshop highlights the range).
- By default the histogram displays the values of your entire image, which includes all layers.
- To display the histogram of a single layer, choose Selected Layer from the Source popup menu. Photoshop displays the histogram of the layer that’s currently selected in the Layers palette. If the image doesn’t have layers, the option isn’t available.
- To show the histogram of an adjustment layer, select Adjustment Composite from the Source pop-up menu. Photoshop displays the histogram of the adjustment layer that’s currently selected in the Layers palette (and all the layers below that adjustment layer).
- Be sure to select any Preview options in the dialog boxes of your image adjustments, such as Levels. That way, the Histogram palette displays both the original and adjusted histograms.
- The Cached Data Warning icon lets you know that Photoshop is reading the histogram from cache instead of your image’s current state. Cache is a reserved, high-speed section of your computer’s memory. The image cache allows the histogram to display faster because it is calculating the histogram based on a representative sampling of the pixels in your image. If you want to refresh the histogram to display all the pixels in your image in its current state, click the Uncached Refresh button. You can also choose this command from the palette pop-up menu. Or simply double-click anywhere in the histogram. Unless it is really bogging down your workflow, I recommend viewing your image’s histograms using uncached data.
Select the Use Cache for Histograms option in the Memory and Image Cache panel of your Preferences if you wish to utilize cached image data in your histogram.
Understanding the histogram statistics
The Histogram palette gives you all kinds of statistics about the pixels in your image. Some of these statistics, such as Standard Deviation, may be for those who live in the land of Calculus. But you may be able to glean some useful information from some of the other statistics that can help you in your image-adjusting tasks. Here’s a brief explanation of each statistic:
- Mean: Average intensity value
- Standard Deviation: How much the intensity values vary
- Median: Middle value of the intensity value range
- Pixels: Total number of pixels used to represent the histogram
- Cache Level: The current level of image cache used to calculate the histogram
As mentioned in the preceding section, when the histogram is calculated using image cache it is basing it on a sampling of the pixels in your image. The original image is cache level 1. For every level above that, Photoshop averages four adjacent pixels below to get one pixel, so each level is half the dimensions (1⁄4 the number of pixels) of the level below. When you choose the Uncached Refresh command, the cache level reverts to 1.
The next three statistics display a value only when you position or drag your cursor in the histogram. Each value corresponds only to the portion of the histogram under your cursor.
- Level: Intensity level
- Count: The total number of pixels corresponding to that intensity level
- Percentile: The number of cumulative pixels (in percentages) at or below that level, from 0% (left) to 100% (right)
Enhancing Images with Adjustments in Photoshop
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