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Stealing Contrast with Photoshop Curves

There's only a limited amount of contrast to go around — 256 levels in 8-bit mode or the equivalent range with greater precision in 16-bit. If we haven't set the black point or white point yet, some of this contrast may be lying unused, available for the taking, but once the brightness range of an image spans from 0 to 255 we have to be more creative to solve contrast problems. We need to look for ways to redistribute the wealth of contrast we have to where it is needed the most.

Thankfully, we have Curves available to help with the task at hand. While the examples shown here were done in Photoshop CS, they will work just as well in Photoshop Elements, provided you have added Curves support as discussed week before last.

Shown below is a shot taken at Sprague Lake in Rocky Mountain National Park just past sunrise. The view is looking west, away from the rising sun, towards Hallett Peak. The tops of the mountains are well lit and fairly bright as is the reflection off the lake surface, while the trees along the shore are still deep in shadows. This sort of extremely high contrast scene is not uncommon at this time of the morning. Even though I used a graduated neutral density filter over the mountain peaks as well as a second inserted from the bottom over the water, I still don't have the detail in the trees I was hoping for. I could see plenty of detail with my eyes, but the camera just couldn't capture it. Let's see what we can do to fix this in Photoshop.

Looking at the histogram for this image shows that it has a great many pixels with data in the quarter tone shadow area as well as in the three-quarter tone highlight region. There are only a few pixels with values in the mid-tones. If we are looking for a way to get more detail in the trees we might think to move the midpoint slider in Levels towards the left, but this results in a fairly washed out image and sacrifices much in the highlights, even when the blending mode is set to Luminosity. The problem is that this increases contrast most in the mid-tones, the area that needs it least here.

Levels is basically a watered down version of Curves with only a single control point in the middle. Real Curves allow us to place any number of control points where we need them. Since slope equals contrast, we can place points so as to increase slope in the shadow and highlight areas, stealing slope from the mid-tones since they aren't needed as much there.

Hallett Peak and Sprague Lake
Levels show lots of data in the shadows and highlights detail, but no mid-tones
Lots of data in the shadows and highlights detail, but no mid-tones
Adjusted via Levels
Adjusted with Levels washes out image, particularly highlights
Levels washes out image, particularly highlights
Adjusted via Curves
Adjusting with Curvess yields much better shadow detail without sacrificing highlights
Histogram after adjusting with Curves shows much better
shadow detail without sacrificing highlights

Let's look at this idea more closely including the curve used to create this result. The initial "non curve" diagonal line Curves starts with maps input values to output values in a direct one to one fashion. Each input value between 0 and 255 maps to itself as an output value, resulting in the diagonal shape. If we want to know where along that line any given point in an image falls, Curves gives us an easy way to tell. Holding down the left mouse button turns the regular cursor into an eyedropper. As we move this eyedropper over the image, the point on the curve mapping that brightness value will appear as an open point. Doing so with the image here confirms that much of the image lies either towards the shadow or highlight ends but not in the middle.

If we hold down the Control key while left-mouse-clicking on the image we'll actually leave a point behind on the Curve that we can use to adjust it. Shown below on the left is the initial Curves diagonal with a number of points marked to show where they lie. You wouldn't normally do this, but I've done so here for purposes of illustration. Your results will generally look better with fewer points since it can be difficult to adjust one with too many points and still get a smooth curve in the end.

Where all our data lies
Control points showing where all our data lies
Sample point eyedropper
Sample point eyedropper
Curves used for final image
Curves used for final image

Let's start over with a virgin Curve diagonal (or we could also delete all the test points by dragging them off the grid). Moving the eyedropper over the trees shows that their brightest area is somewhere around the quarter-tone area (the top of the first of four blocks in the default grid. If your Curves grid is divided into 10 sections instead, you can toggle to the four-division quarters view by clicking on the grid with the Alt/Option key held down. To allow us to increase the contrast (slope) of the portion up to this point, we can click on the diagonal at this point. We could also Control click in the brightest area in the trees to create the same point.

We can do similarly with the mountain tops to find that the darkest area we care about lies somewhere near the bottom corner of the top block, the three-quarter-tone area. Placing a point here will let us adjust contrast in the highlights separately from mid-tones.

We can now move our first point upward to simultaneously increase slope in the shadow portion of the curve while decreasing it in the mid-tones. We have effectively stolen contrast from the mid-tones where not much of the image lies and moved it to the shadow and quarter-tone area where the trees desperately need it. Having the control point at the top of the mid-tones holds that point in place on the curve, limiting the slope decrease to just the mid-tones. If this curve hadn't been there, the Curve would have smoothly gone from our quarter-tone control point on up to highlight end of the curve, lessening contrast in the sky and mountain peaks where we wanted to keep things as they were.

Placing and adjusting control points is a process of analyzing the image to see where we need contrast and where we can steal some, along with visually watching the image as we move points around until we see what we are after. In the end, I ended up with the two Curves points shown above on the right. This gives us a much more satisfactory image that holds contrast in the highlight area, while shifting some from the mid-tones to the shadow area where the trees are. By the way, if you haven't already guessed, it's a good idea to use Curves on an adjustment layer so you can fine tune things later if need be.

Robin Hood, the figure said to have lived in medieval times in Sherwood Forrest with Little John, Friar Tuck and the other Merry Men is famous for his efforts at redistributing wealth, stealing from the rich and giving to the poor. Photoshop Curves can be seen in much the same light, allowing us to "steal from the contrast rich and give to the poor" portions of an image. Let's hope the Sheriff of Nottingham doesn't find out.

Date posted: January 16, 2005


Copyright © 2005 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: Curves, Curves, Everywhere Curves Return to archives menu Next tip: Color Management Answers for Photoshop Elements

Related articles:
Essential Filters: Neutral Density and Graduated ND
Your Friend, the Histogram
The 1-2-3 of Photoshop Levels
Photoshop Curves: Stepping Up From Levels
Curves (and Other Goodies) for Photoshop Elements
Shadows and Highlights, the Easy Way
Some Random Thoughts on Superman and Photoshop Curves

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