Once you have your image saved on the computer, you can manipulate it in an image editing program such as Adobe Photoshop®. As you are working on your images, you should keep them in RGB, some filters don't work in CMYK and switching back and forth between RGB and CMYK loses valuable color information that you can't get back. Convert to CMYK the last thing you do.

Editing

When working with color, use the neutral gray pasteboard around your image. If you have your computers wallpaper, or desktop in view while working on your images, the color perception of your image will be distorted.

You can edit the entire image or make a selection to edit only a part of the image. The easiest way to make a selection is to use the Marquee Tool, which restricts the selection to either rectangular or circular. Other options for making a selection are to use the magic wand tool to select a specific color, the lasso tool to draw a freehand selection, or to use the pen tool. You can use the pen tool to create a path which, after it has been saved, can be activated at any time.

Steps to adjust your image:

• Calibrate
• Check the scan quality
• Set the highlights, shadows, and midtones
• Do any airbrushing 
• Apply any filter effects
• Apply Unsharp Masking
• View Proof

Calibrate your system

The most critical part to any color work on any system is to make sure everything is calibrated. Otherwise the image on the screen could look totally different once it is printed. You need to calibrate your system often to ensure that what you see on your screen will be the same on the printed product. For more information see color management.

Scan Quality

You want to make sure your scan has enough color information to produce a high-quality image. The higher the number of pixels in an area, the greater the detail. Bad photos to begin with or bad scans may be impossible to correct.

You can check the histogram located under the Image menu in Photoshop. The histogram shows the number of pixels at each brightness level in an image. It also shows if there is enough detail to make a good correction and gives a quick picture of the tonal range of the image. If there are white gaps instead of solid black, there is missing data.

Highlights, Shadows and Midtones  

There are several different ways to set your highlights, shadows, and midtones. You can use either the Levels palette or the Curves palette, whichever you prefer. The first thing to do is to set the white and black (highlight and shadow) points and then adjust the midtones.

To set the highlight and shadow, use the eyedropper tools available in either the Levels or the Curves palettes. First double-click the highlight dropper (white one on the right) and set what color you want your highlight to be. Then pick the lightest point in your image by looking at the Info palette. Next, double-click on the shadow dropper (on the left) and change the settings there. Then, pick the darkest point in your image. If you do not know what values to use, ask your print provider what they suggest. Typically for printing an average image on white paper, you want to set the highlights for CMYK to the values of 5, 3, 3, 0; or RGB values to 244, 244, 244; or grayscale to 4%. Set the shadows values for CMYK to 65, 53, 51, and 95; or RGB to 10, 10, 10; or grayscale to 95%.

Another way is to use the Levels palette. You slide the highlight end point (right side of the histogram) to the left or the shadow end point (left side) to the right, or slide the middle one either direction to adjust the midtones.

One more way to set the highlight and shadows is through the Curves menu. Here you adjust the curve until it is the color you want. 

Once you have the highlight and shadow points set, the midtones should be fine. If you still need to adjust it, you can use either the Levels palette or Curves palette. In the Levels palette, slide the middle triangle to adjust the midtones until correct. You may also add up to 14 points on the grid in order to adjust any other tones in the image. Shown below is the Curves palette used to lighten the midtones while working in RGB mode.

Beginning color

End color

Process

Air brushing

The clone tool can be used to air brush out parts of the image that you want to get rid of or to duplicate parts of the image. Pick the rubber stamp tool (or clone tool). Hold down the option key and click the area where you want to duplicate. Then click in the area where you want to do the painting and either keep clicking the mouse button or hold and drag, whichever gives you the best results for the type of image you are working with. 

Beginning

End

Process

Filters

Filters are applied to bitmapped images to create special effects. Some filters come with the image editing programs, or there are a lot of third-party filters that can be added. Here are some samples of some of the filters available in Adobe Photoshop.

Original Image	Chalk & Charcoal	Colored Pencil
Craquelure	Find Edges	Ocean Ripple
Poster Edges	Rough Pastels	Spatter

 
Scaling and Resampling

Before you start to make changes in the image size box, make sure to check the Constrain Proportions check box. The chain link next to the width and height boxes also refers to being in proportion. If don't want to scale proportionately, uncheck the box, but remember you will be distorting the image.

If you know the percentage that you want to change the image rather than the size, click on the drop down boxes next to the sizes and change it to percentage.

Before you scale the image, you should understand resampling. When you change an image's file size, the software automatically adds or subtracts pixels in your image. Changing the image's file size is called resampling. If you decrease the dimensions of your image size, pixels are subtracted; if you increase the image size, pixels are added. When pixels are added, they are interpolated, a process where the software adds more pixels and then colors them with the average color of surrounding pixels. Interpolating can result in a somewhat blurred or fuzzy image.

There are three possible methods of resampling. The default is Bicubic, which is the best possible method, but is also the slowest. It attempts to improve contrast while interpolating. Nearest Neighbor is the fastest but least exact interpolation method. If you use the Nearest Neighbor method, your image will probably look jagged after rotating or other manipulation. Bilinear is in between Bicubic and Nearest Neighbor.

With resampling checked...

• If you decrease the Print Size, the image file size and the number of pixels in the image also decrease. The Resolution setting does not change.
• If you increase the Print Size, the file size grows and the number of pixels in the image increases. The Resolution setting does not change.
• If you decrease the Resolution, the file size decreases. The Print Size does not change, but the number of pixels in the image decreases.
• If you increase the Resolution, the file size increases. The Print Size does not change, but the number of pixels in the image increases.

If you want to change the Resolution or the Print Size without resampling, uncheck the Resample Image check box. Without resampling, if you change the Print Size dimensions the resolution will also change, or by changing the resolution, the Print Size will also be changed.

With resampling unchecked...

• If you decrease the Print Size, the file size does not change. The Resolution setting increases.
• If you increase the Print Size, the file size does not change. The Resolution setting decreases.
• If you decrease the Resolution, the file size does not change. The Print Size is increased.
• If you increase the Resolution, the file size does not change. The Print Size is decreased.

Unsharp Mask (USM)

Unsharp masking is the last thing that should be performed on an image. Always save a copy of the file before peforming the unsharp masking step. It is hard to judge the outcome of unsharp masking on the monitor. You will need to do a proof to see the final results.

• Amount - Amount determines how much to increase the contrast of pixels, how much darker and how much lighter the edge borders become. The amount entered can be from 1-500 percent, with the greater the number, the greater the sharpening. High-resolution images require a higher percentage. Using 150-300% usually provides good results with high-resolution images.
• Radius - Radius specifies the distance out from the transitional edge that you want sharpened. A lower value sharpening occurs closer to the image edges, a higher value sharpens a wider band of pixels. The suggested value for high-resolution images is between 1 and 2. Small detail such as human faces can tolerate the least amount, or else any lines and wrinkles would be more exaggerated.  Large objects with large detail can use the most, and landscapes are somewhere in between.
• Threshold - The threshold value determines how much contrast there need to be in order for it to be considered an edge. Lower numbers sharpen more because fewer areas are excluded. The default 0 sharpens all pixels in the image. Higher values exclude areas of lower contrast.

View Proof

Color is affected by the type of light in which it is viewed. Different lighting conditions will change the appearance of the color on a proof. For example, the colors on a proof will look different under fluorescent lighting than when viewed under sunlight. The colors surrounding the proof will also affect the colors of the proof. Standard viewing conditions have been established within the graphic arts industry because of the affect that different lighting has on viewing colors. In order to accurately view color, a color viewing booth is a necessary item. A proper booth should use 5000 degree Kelvin lighting (standard daylight) with equal color temperatures in each part of the spectrum, and no reflections from surrounding surfaces.

Click here for more information on proofing options.