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Introduction to Photoshop: Editing Images

This guide introduces important concepts in digital imaging and techniques in Adobe Photoshop.

Created by Health Science Librarians

Editing Images

Editing Images

Photoshop provides numerous powerful tools for working with images. This section describes key tools for basic image editing.

Keep in mind the following when editing images:

  • It is always a good idea to keep your original image as a backup and work on a copy, in case you make a mistake and want to start over.  (If you want to go back to the file as it was when you opened it, choose File>Revert. You can also go back to a previous state of your image by  using the History palette.)

  • If you have selected an area (using the selection tools), adjustments you make affect only the selected area. Otherwise most adjustments affect all the pixels in the image.

  • If your image has multiple layers, adjustments you make generally affect only the current layer. (The exception to this is adjustment layers).

Cropping Images


1. Choose the Crop tool () in the toolbox. Then click and drag across the image. An outline will appear that shows you the area to be cropped.



2. Adjust the area to be cropped by using handles on the edges of the image.

Note: If you move the cursor just outside of one of the corner handles, it turns into a curved cursor, which then allows you change the alignment of the selected area. This is useful, for example, to correct a scan of a picture that was in a crooked position on the  scanner.

3. Press Enter to crop or Escape to cancel

Adjusting Contrast

Adjusting Contrast

The Levels command (Image>Adjust>Levels) is an excellent tool for adjusting contrast. The Levels dialog box displays a histogram of the brightness values of the pixels in your image. There are two sets of sliders (triangles): input levels and output levels. For most editing tasks, use the input levels sliders (the three triangles just below the histogram).

  • To increase contrast (create more dark and light pixels): Move the left and right sliders toward the middle.

  • To adjust the midtones: Move the middle input levels slider to the left to lighten midtones, and move the middle slider to the right to darken the midtones.

  • Keep the Preview box checked in order to see the effects of moving the sliders on your image.

  • To make the adjustment, click OK. To cancel it, click Cancel.


In the above example, there are few (if any) pixels that are white or even light gray. By moving the right slider towards the middle, we can lighten image to correct the exposure. Because the Preview box in the layers palette is checked, we can see the result on our image as we adjust the sliders.



Note: In the above example, the Levels dialog box Channel popup menu is set to the default of RGB, which means the histogram represents a combination of all of the channels (Red, Green, and Blue light) in the image. You can use Levels on an individual channel, but this is likely to change the color balance of the image, so in most cases is not likely to be a good idea.

Note: The Levels command is often a good way to get a more even distribution of briightness of pixels. But this is not always desirable. For example, a properly exposed photograph of something dark such as a coal mine whould be dark, and an image of a snow scene would be white. In such cases the histogram would be lopsided.  In such a case there may be no need to make an adjustment in the Levels dialog box.

Levels Adjustment Layer

For more control over a levels adjustment, consider using a levels adjustment layer (Layer menu > New Adjustment Layer > Levels...).  A Levels adjustment layer allows you to adjust levels nondestructively.  You can reopen your Photoshop file and change the layers adjustment in your Photoshop file whenever you want.


Color Correction

Color Correction

Often you may need or want to change the balance of color in an image. For example, a photo take outdoors may look too blue, or a photo taken indoors may look yellow or orange.

There are several ways to adjust color, many of which are under Adjustments in the Image menu, including Color Balance, Color Mixer, and Photo filters. These tools are easy to use and the best way to learn about them is to open a practice image and choose one of these commands, and experiment with the settings in the dialog box that appears for the command. 

Auto Color

The easiest color correction tool is Auto Color. Auto Color often (though not always) gives very good results. Auto Color attempts to color correct images automatically by analyzing the highlights, shadows, and midtones of each channel.

Choose Image>Auto Color to use the Auto Color command and see if you like the results.

The following is an example of an image before Auto Color was applied. The image is good but slightly more blue than I would prefer:



The following is the image after applying Auto Color.  The difference is subtle, but Auto Color added a little warmth to the image.


Removing Imperfections

Removing Imperfections

Using the Clone tool

The clone tool ( ), also called the rubber stamp, allows you to remove imperfections such as dust and scratches by covering them with samples of nearby pixels. For best results, zoom in close and use a small brush.

1. Zoom in close to the area you want to edit.

2. Choose the clone tool, and make sure "aligned" is checked in the Options bar.

3. In the options bar, choose a brush size (this determines the size of the sample). The cursor shows the size of the brush, as shown below. The circle is the cursor (brush).

(Note: if the cursor doesn't show the brush size, go to Edit>Preferences>Display & Cursors, and choose Brush Size for Painting Cursors.)

4. Alt-click to define initial source point (sample).

5. Move the cursor over the area you want to cover, and click to cover it with the sample. 

Using the Healing Brush Tool

The steps for using the healing brush tool ( ) are the same as for the clone tool. The healing brush attempts to take the texture from the sample without affecting the brightness of the area to which it is applied. In some cases this is more effective than the clone tool.



Use Filter>Sharpen>Unsharp Mask to sharpen an image. In the dialog box that appears, keep the preview box checked. This allows you to see the effects of different settings.


Amount -- How much sharpening is applied. Consider using a value between 50% and 150% for typical images.

Radius -- The number of pixels affected around edges. Consider using a value between 0.2 and 2.0. (Deke McClelland recommends using 0.1 of radius for every 15 ppi in the image. For example, for an 150 ppi image, you could use a radius value of 1; for 300 ppi, you could use a radius of 2.)

Threshold -- Defines what brightness difference qualifies as an edge (0 is default, sharpens all pixels). Consider using a value somewhere between 1and 5 (for Threshold, the higher the value, the less pronounced the sharpening effect).

Note: Sharpening is more an art than a science, and is somewhat subjective. The above settings are only suggestions. For best results, you need to look at the particular image and figure out for yourself what looks best!  Make sure that in Photoshop the image is at 100% magnification, and keep the Preview box checked in the Unsharp Mask dialog box .