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JMG Galleries Photography by Jim M. Goldstein |
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Unless you like being generic it is essential you understand what Levels and Curves adustments provide. As you become familiar with the information and techniques provided in this article, you too will begin to see over time what a trained eye will instantly see when viewing images that have not been color corrected. A trained eye will instantly see areas of improvement such as color casts and/or muddled higlights/shadows. 
How To Make Adjustments Option 1: Levels/Curves via the Image Menu (See image to the left) In Photoshop or an equivalent piece of software you can edit levels via the Image menu. (Image > Adjustments > Levels) This is a satisfactory method, but a second and more flexible option exists. Option 2: Levels/Curves via the Adjustment Layers menu on the Layers palette window (See image to the left) This method is the most flexible of the two as you can easily undo, compare or change layer settings. I'm embarassed to say I learned about this late in the game and it is a real time saver. |
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Selecting Levels & The Histogram
Levels: Allows you to adjust the tonal range of an image with the use of a histogram.
Is a graph of the number of pixels at each brightness level in an image. Pictured to the right is an unaltered image as captured from a digital camera (see the background behind the histogram window). The histogram, the odd looking graph, designates shadows on the left and true black is represented by a value of 0, while highlights are on the right with white being represented by a value of 255. The arrow in the middle represents the mid-tones and can be adjusted from left to right to either darken or lighten the midtones. The first step of this process and the most important step is to keep in mind the lighting scenario for your photo. Clicking on "Auto" will tell the software to take its best guess at what it thinks the scene should be. This will wash out "magic hour" lighting from early morning or late evening and will devastate a sunrise/sunset. It is always best to manually adjust levels unless you don't care.
Adjusting the histogram to color correct your image can be done for the entire RGB spectrum and on an individual basis for each color channel (Red, Green, and Blue). In evaluating histogram data I look for plateaus of unused data. In this case (see the "Levels Unadjusted" example above) the highlights were severely under represented. Look at the right side of the histogram. I brought the highlight arrow to the beginning of where the data is really starting to register. I also brought in the shadow arrow to darken the shadows as they were a little light. Next use the pull down menu to select each color channel and repeat the above steps. To the left are screencaptures of the adjustments I made for this particular image. This takes some learning by experimentation and compressing the histogram does not always provide the best results. This again depends on the lighting conditions in your image. The example image to the left "Blue Channel Re-Adjusted" is after adjusting the red, green and blue. The histogram only shows the blue channel. I ended up backing off compressing the histogram for each channel to give a more natural appearance to the image. How each channel ended up after a little trial and error, but with some decent educated guesses can be seen to the left in the example image "RGB Final Look". If you go back to the RGB setting after adjusting each color channel you'll see that the composite histogram is quite different now. If you've ever used "Auto Levels" then you'll have seen this transformation to the histogram before, but now you'll know how it's actually put together. |
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Selecting & Using Curves
Allows you to adjust the tonal range of an image in an alternate fashion with the use of an Arbitrary Map versus a Histogram. Curves can be found just like Levels as detailed above in the How To Make Adustments portion of this article.
An Arbitrary map (see the images above) displays a line with the lower left end representing shadows, the upper right end representing highlights and the middle representing midtones. To adjust an image using the Arbitrary map you click on the line to set a point and drag it around. You can designate up to 16 points to set and adjust. If you click a point on the far left it will represent what should be true black and if you click a point on the far right it will represent what should be true white. Dragging these points subtly can provide dramatic improvements. Dragging these points drastically can create wild distortions. The result of tweaking the black values to an extreme can be seen in the image to the left titled "Shadows RGB". Note if you shifted this point up versus to the right reversing the values "Output: 58" and "Input: 0" the shadows would become much lighter versus much darker. The result of tweaking the white values to an extreme can be seen in the image to the left titled "Highlights RGB". Note if you shifted this point up versus to the right reversing the values "Output: 187" and "Input: 255" the highlights would become much darker versus much lighter. Throwing a mid-tone value into an extreme placement will result in changes exemplified in the images titled "Dark Midtones (RGB)" and "Light Midtones (RGB)" to the left. |
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The final results of adjusting Levels and Curves in this example can be seen in the Before & After image to the left. On the left of this image is the Before (no Levels or Curves Adjustments) and on the right is the After (with Levels & Curves Adjustments). |
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