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Levels corrections are the easiest way to get decent color quickly. It is a correction you can make almost without checking the image visually first (although it is always a good idea to make a visual check of any image you are correcting). The exceptions to going ahead with Levels corrections as described are for images that you know have desired color casts (such as sunset or sunrise pictures, in which there will be a decided and necessary color shift). Doing this correction to images with known color casts will probably ruin the desired tone and color effects.
WARNING
If you use this technique, be careful when you encounter unusual histograms, or large gaps or tails in the highlight or shadow area in any of the image color channels. In this case, the histogram can be considered the graph in the Levels palette.
NOTE
A histogram is a graphic representation of the way color is distributed in an image.
Tail refers to a histogram attribute in which image information is present and measurable on the histogram, but appears as a thin line and may be image noise rather than detail.
Histograms with long tails, or images in which one or two of the three colors seems extremely compressed, may suggest that there is some intentional distortion, such as the use of a filter by the photographer. It also may suggest that you have a bad scan or other digital representation. This Levels technique corrects quite nicely for fluorescent and most other man-made lighting.
The more extreme the changes you make in the Levels correction, the more chance you will have to damage the image. If you have a long tail to correct, do it a little at a time (make two or three separate corrections, closing out of the Levels in between). Some experience doing these corrections will help you know what to do, when to use more than one correction, or when to leave well enough alone.
Correcting for a lengthy tail in any histogram may produce poor results if you choose to cut off the tail completely. As a general rule, the longer the tail, the less likely it is that you should cut it off; cut half or a quarter off if the bulk of information in the channel seems compressed. If the information in the channel is very compressed (with the bulk of the information falling in one-third or less of the potential range), meet the tail only partway.
The middle ground is a better way to go in corrections, especially if only one of the channels has such a tail. If all channel histograms look similar, do approximately the same thing to each of the channels. If all look very different, you will have to make a judgment call, based on visual examination of the image. Experience helps with this type of evaluation and correction, so the more you do, the easier it will become.
Figures 3 and 4 show the RGB levels and Red level for the photo shown in Figure 5. Follow the lead of the correction shown in Figure 4, and complete the correction on the image in Figure 5 by changing the Green and Blue levels as well.
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This image has histograms that show a significant tail in the highlights of each of the channels—shown here in the composite RGB. Depending on how the image looks, you can choose to cut the tail, make a conservative correction, or meet the cast halfway.
Depending on the reaction of the image, it is OK to cut off a tail, even if some image information will be lost. Be aware that this will increase the image contrast. To cut the tail, move the slider in, and crop by selecting OK.
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These two figures show a comparison of the Level correction technique applied. The original image isn't awful, but the corrected version has better contrast and more intense color. Download the image and try the correction yourself.