Contrast adjustment is the process of adjusting light and shadow in an image. The problem is that the world offers a very large range of tones, from the darkness of peering into a cave to while clouds in bright sun. The print on paper or view on a computer display has less capability to show a range of tones. Even if our display could exactly match the real world, we still might like to cheat and not show blinding glare from shiny objects or let objects be hidden in shadow -- as they are in the real world. The challenge of contrast adjustment is to present the tones in an image is a pleasing way.
An Extreme CaseWe will begin with an image that has serious contrast problems. This image of a market in Australia was taken on film in low light and later digitized. The first step is to open the image in Photoshop Elements Full Edit:
The areas that should be black, or nearly black, are grayish. We can get a substantial improvement by selecting Enhance > Auto Levels. this adjusts the tones so that lowest level in the picture is completely black and the brightest white is full white:
Often, just using Auto Levels will be enough to fix a contrast problem, and if so there is no need to press on with further tweaking. Here, the part of the market in the distance is a little dark. Selecting Enhance > Adjust Lighting > Highlights/Shadows ..., we set the slider so that shadows are lightened 10% and highlights are left without darkening (0%):
Lightening shadows or darkening highlights is the next most common adjustment, next in usefulness to Auto Levels.
The mid tones are fairly good, but the black areas could be made a little more black. Use Enhance > Adjust Lighting > Levels and move the black level arrow under the graph in the pop up window slightly to the right. This yields:
The general sequence of first using auto levels, then adjusting highlights/ shadows and finally tweaking the levels is a good pattern for contrast adjustment.
Punching Up an ImageThe next case is an image that is not so bad to start with. Here is the original, taken near a park in the San Francisco area, with the graph brought up by Enhance > Adjust Lighting Levels:
The graph is called the histogram of the image. It shows how the values of light and dark are distributed in the image. Darker tones are to the left and light tones to the right. An histogram of a black and white checker pattern would have only a spike on the left end (the black squares) and a spike on the right end (for the white squares). An image with no pure blacks would have the graph shifted to the right so that the part of the graph near the left end is zero.
Let's see what Enhance > Auto Levels does to the image:
The Auto Levels logic decided the image needed more contrast, so it shifted tones away from mid-tone region towards the bright and dark ends of the histogram. If you compare the histograms carefully, you'll see that the peaks on the bright end (the right) are shifted to the right, towards pure white. There is also more mass near the dark end. Shifting tones outward has left some of the middle tones less used, so there are narrow gaps in the middle. The graphs are scaled in height automatically, so it is only the shape of the curve comparing the left to the middle to the right that counts.
The sky seems a little weak. We selected the sky from the top of the frame down to about the level of the hills using the selection tool from the tool bar. To obscure the line between adjusted and not-adjusted areas, we used Select > Feather > 30 pixels. finally we made the adjustment to the sky using Enhance > Adjust Lighting > Brightness/Contrast. The brightness was lowered by about 40 steps and the contrast increased by 40 steps:
One trick remains. To enhance the illusion of depth, increase the contrast of the near foreground. this plays to the psychology that the distance ought to be a little more hazy, or less contrasty than the foreground. The procedure is the same as used with the sky, selecting, feathering, darkening, and increasing contrast. The resulting histogram is shown. We picked up a spike of pure black, and that caused the software to scale the histogram down. The total number of tone values is really still the same.
Here is the final image:
It is punched up from the original. I am sympathetic to a degree with the argument that it is overdone -- too artificially dramatic compared to the original. I think it is acceptable for this image because it was a rather dramatic scene in real life, but I don't deny that there are other images where adding "too much drama" degrades them. High impact images invariably win photo contests and get high marks on flickr. The simple technique of underexposing by about one-half to one stop seems to please many people.
SummaryTo summarize, if a picture looks unpleasantly hazy or dull, follow these steps, quitting as soon as you find the picture acceptable:
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