Main Points

There are two distinct styles of using Photoshop; you can use Photoshop as a graphic design tool much like people use Illustrator. There is quite a bit of overlap between these two products when used for graphic design and some prefer one or the other, while others find they like to use both choosing one or the other for specific tasks. The second way of using Photoshop, however, is uniquely Photoshop, as an image editor. And if you are editing images, your best friend is the Curves Adjustment layer. Understand that there's no reason that you can't use Curves on graphic designs, but let's say that most people don't; it's usually used with images. This is an amazingly powerful tool that gives you the ability to exercise precise control over light values which is all your image really is. It is somewhat complicated, but like the Pen tool in Illustrator, grappling with the learning curve is a requirement to get good results out of your images.

There are numerous other adjustment layers you can apply which mimic small pieces of what Curves does, and almost all of them don't look as good as if you'd used Curves for those tasks instead. Curves also provides a much more systematic way of approaching image editing. I've frequently heard people give the following advice for Photoshop image editing, "Apply the adjustment that will make the most difference first." Ever been annoyed that you weren't sure or wound up wasting time trying to figure it out which adjustment layer that was? The fact is, that's not a very scientific way of approaching things. While this approach tries to make the best of the confusing hodge podge of choices, Curves offers a much more progressive, step-by-step workflow. That lets you not only produce better results, but do so faster, and you'll love that.

Curves Explanation

The Curves tool is very powerful. The Curves tool is very easy to use. The Curves tool is maybe not so easy to understand. Especially if you had problems with graphs in school, for the Curves adjustment controls are nothing more or less than a graph. The bottom border (the x-axis) represents the different light values of your image. On the left is a value of 0 light, black and on the right is 255 light, white; these are the minimum and maximum values for light in the RGB color system that's used on computer monitors. The left border (the y-axis) represents the different light values you Want in your image. On the bottom is a value of 0 light, black and at the top is 255 light, white. So the bottom border tells you what light values you have, and the left border shows what you are changing them to. Just like graphing, each point on the Curves graph corresponds to two values, one value on the bottom border (the input value from the original image) and one value on the left border (the output value you want to change it to).

Next, please notice there is a diagonal line, it's perfectly straight at the moment going from the bottom left corner to the top right corner. If we can stop and consider this for a second, it's really pretty simple. Each point on the line corresponds to one number on the bottom border (which is a light value in your original image) and a second number on the left border (which is the light value that it has been changed to). Therefore, the line shows you what changes you have (or in this case haven't) made to the light values in your image. Here is the key concept to wrap your brain around when you start working with Curves: this straight diagonal line represents "no changes" to the light values of the original image. This makes perfect sense when we break it down. The starting point of the graph is at the bottom left corner (0, 0) which indicates that for an input value of 0 light at the bottom border there is no change; therefore it corresponds to an output value of 0 light on the left border. The ending point of the graph is at the top right corner (255, 255) which indicates that for an input value of 255 light at the bottom border there is no change; therefore it corresponds to an output value of 255 light on the left border. Each point on the line represents this same relationship, all points on the line have the identical output value to the input value indicating that no light values have been changed.

Once you've grasped that the straight diagonal line indicates no changes, you can begin to understand what it means to start using Curves adjustments. When you drop a new point onto the graph, the curve automatically adjusts to move as smoothly as possible through your point. Look at the new points along your curve. If the output value on the left border is more than the input value on the bottom border, you are brightening any pixel in the image which has that light value; specifically you have brightened it from whatever its original input value to the new output value so the larger the change, the brighter you are making it. Contrarily, if the output value is less than the input value, you have darkened any pixel in the image with that light value - and darkened it from the input value to the output value. These cases are very simple to see visually. Since all points on the starting diagonal line represent no change, any portions of the curve you drag above the original line will be lighter and any portions of the curve you drag below the original line will be darker. Those are the basics of the Curves adjustment tool.

Advanced Curves Concepts

OK, so now you have the Curves basics under your belt. You can certainly play around with using them, and for some images that's all you need to know. However, in order to use Curves quickly and expertly, so you can get on with your image editing, you have to recognize and control curve patterns. This ability to see and manipulate the patterns of the Curves adjustment is what makes it so superior to the slider control tools; it gives you much more precise control. It's also so visual that once you understand it, you have so much more information about your image to work with. That allows you to make faster and better image editing choices.

    Basic Patterns

  1. 1/1 "Curves": We discussed how moving the curve above the starting line made pixels in the image brighter and moving the curve below the starting line made them darker. However, there are also patterns in the slope of the line itself. The starting line represents one particular "normal" curve, and a pretty boring one at that. Any part of the curve with roughly the same slope will have similar effects on the image. If you remember slope from algebra, fantastic. If not, slope is how steep the curve is. The slope of the starting line is 1/1, ie if you increase the input light value by one, the output light value also increases by one. Any part of the curve with roughly 1/1 slope will preserve the contrast of the original image.
  2. Steep Curves: If a curve becomes steeper than the starting line (steeper than 1/1), then you increase the contrast; when the input light value increases by one, the output light value increases by more than one. The steeper the curve, the more contrast there is in those range of light values. In short, where the curve is steep, two input values which were so close as to be hard to tell apart become two output values that are far apart and noticeably different, ie higher contrast.
  3. Flattened Curves: If a curve becomes flatter than the starting line (flatter than 1/1), then you decrease the contrast; when the input light value increase by one, the output light value increases by less than one. The flatter the curve, the less contrast there is in those range of light values. In short, where the curve is flattened, two input values which were far enough apart to be noticeably different, become two output values that are very close together and hard to distinguish, ie lower contrast.

You now know enough about Curves to control brightness and contrast much better than you ever could using the Brightness / Contrast adjustment, thus eliminating the need to ever use it again. If you want to brighten the image drag the curve up, if you want to darken it, drag it down. Moreover, Curves lets you easily lighten some portions of the image, while darkening others; something impossible with the brightness slider. If you want to increase the contrast, make the curve steeper, and if you want to decrease the contrast make the curve flatter. Moreover, Curves lets you increase contrast in some portions of the image while decreasing the contrast in others; something impossible to do with the contrast slider. In fact, in the Curves adjustment, you can see that any part of a curve you make steeper will force another part(s) of the curve to be flatter and vice versa; since you have to get from 0 to 255 on each axis, if you flatten part of a curve, other parts will become steeper to compensate. Making these adjustments on the Curves graph lets you carefully balance the tradeoffs for how steep each portion of the curve should be to get the best looking picture; this is one of the most sensitive adjustments you need to make in image editing for which Curves is light years ahead of dragging the contrast slider back and accepting the limited selection of choices it gives you.

    Professional Patterns

  1. The "S" Curve: This may be the single most common curve that people apply in image editing. The "S" curve starts out flat at the bottom left, then becomes steeper through the middle and flattens out at the top before reaching the top right corner. You should now be able to tell exactly what this curve does. It sacrifices contrast in the low and high ends making the darkest colors even darker and the lightest colors even lighter because these areas have been flattened. As a direct consequence, the middle portion of the curve becomes steeper and colors in the midrange become more vibrant from the increased contrast in their range. The S curve works successfully on many images because they have few pixels in the very brightest and very darkest areas and many pixels in the middle. If your histogram looks like a bell (short or no bars on the ends, and tall bars in the middle, the "S" curve is perfect. It lets you reduce contrast on a relatively small number of pixels at the ends to increase contrast on the large number of pixels in the middle of the histogram. If you've ever heard someone talk about applying a Curves adjustment to give an image some "pop", this increase in contrast that brings out more vibrant colors is what they're talking about.
  2. High Contrast Curves: Picking up on our discussion of the "S" curve, pictures usually become more vibrant and attractive with higher contrast. The "S" curve works because you've decreased the contrast in the darkest and lightest colors where there are few pixels and increased the contrast where there are lots of pixels; the effect is that the vast majority of pixels have higher contrast. However, you should now be able to see how the "S" curve can fail. If there are lots of pixels at the very light or the very dark end of the histogram, you are no longer losing contrast on a small number of pixels. There is a critical general concept you should take away from this. Wherever the histogram shows you there are tall peaks, make the curve steeper across those areas; wherever the histogram shows you there are valleys or low plains, make the curve flatter. That will allow you to have maximum contrast over the most pixels, which usually makes your picture look much better. For this reason you can see why the addition of a histogram to the Curves adjustment in CS3 was such a major advance.
  3. Downslope Curves: There's something almost everyone experiments with at some point that we haven't mentioned yet. We've talked about steep up curves, "normal" up curves (slope 1/1), and flattened curves. We haven't talked about curves that go down steeply or otherwise. Go ahead and try it. What do you think? When it comes to image editing down is awful, unless you're going for especially jarring, discordant artistic effects. A downslope means that when the input light value increases by one, the output light value actually decreases. This represents a color inversion which produces the strange effect you can see all too clearly. It therefore doesn't provide you with many useful options and explains why curves may flatten out, but they always go up.
  4. Clipping Lines: It is easy to forget clipping. The natural behavior of the Curves adjustment when you start dragging points around is to have a point in the bottom left corner with a curve leading up at a steeper or flatter slope, ultimately leading to a curve with a steeper or flatter slope rising up to the top right corner. It is easy to forget that you can create a flattened portion of the curve at the ends. If you drag a point up to the top border, that input light value and every input value higher than it are assigned a value of 255, white. Contrarily, if you drag a point down to the bottom border, that input light value and every input value below it are assigned a value of 0, black. Doing either (or both) of these things creates a line segment between the black point and the bottom left corner or the white point and the top right. These are clipping lines, and they have the effect of using the white and black sliders of a Levels adjustment. (As a quick note, this basically eliminates the usefulness of the Levels adjustment. The only other ability of the Levels adjustment is the middle gray slider. Comparing the Level's middle gray slider to a Curves adjustment's ability to exactly adjust the slope of the curves to maximize your contrast is like comparing a paper airplane to a Boeing 747). There is a second form of clipping lines, but it tends to reduce the quality of your image so it's not used often. However, it is technically possible for you to create vertical clipping lines by dragging a point against the left or right borders. Instead of having the full range from 0 to 255 possible output light values, you have reduced your available range and therefore reduced your contrast - and usually the attractiveness of your image too, though there are exceptions.
  5. Color Channels: So far we've only discussed the normal behavior if you start clicking around on the graph of the Curves adjustment. There's another powerful tool available, as you have the option to change the color channels you're adjusting. The channel is initially set to "RGB" and everything I've described so far are the effects of using the Curves adjustment on that setting alone. However, you can change the channel from RGB to any of the specific color channels: red, green, or blue. There is an easy comparison here. The Color Balance adjustment is a much more powerful version of the Brightness Control. By moving the red, green, and blue sliders the same amount, you can duplicate the brightening or darkening effect of the Brightness control. However, the Color Balance also allows you to apply a color cast to (or more frequently remove a color cast from) an image. You can even adjust the color cast of shadows, midtones, and highlights separately. This is the function that the color channels of the Curves tool replaces. Just as the RGB channel of the curves tool, replaces the Brightness control, each color channel replaces the color slider of the Color Balance adjustment. And whereas the Color Balance only gives you three steps to apply color adjustments to, Curves gives you the entire range from 0 to 255. So Curves not only replaces the Color Balance adjustment, like the Brightness / Contrast adjustment before it, it gives you much more power to control your results.

There are four major adjustments grouped at the top of the adjustment layers. Curves is one, and you now understand the principles of how use it to replace all three of the others: Levels, Color Balance, and Brightness / Contrast. Typically you'll get better results than you would have with the original because you have much finer control. You will also find that you gain much more expertise and speed editing your images. Because the Curves adjustment performs all the functions of all four of the primary adjustments, if you use it alone, you will be using it all the time and get so much more practice with it (roughly four times as much) as you ever would with any of the other adjustments. That practice translates into more comfort, more confidence, and more speed.

Curves Workflow

One of my favorite things about using Curves is that I no longer have to decide which adjustment is going to make the most difference. I already know that it's going to be Curves. As promised, here are some of my Curves best practices.

  1. Color Clipping: Clipping is typically what you would use Levels for. Levels provides you with a histogram and if the color information doesn't go all the way to the ends, typically you want to move the black and white sliders to the beginning of significantly high bars. If this makes your picture look bad and / or weird, you know you have a special case. Photoshop CS3 included a histogram in the Curves adjustment for the first time, eliminating any real need to use Levels. If there is a black gap (on the left), drag a point to the bottom border at the end of the gap. If there is a white gap (on the right), drag a point to the top border at the beginning of the gap. The gray slider in Levels is one of those things that does a far inferior job to Curves, we'll take care of it as we continue and the picture will look much better than if we'd used Levels.
  2. Brightness: This is typically where you'd use the Brightness slider, except now you don't have to wait for it to open. Grab a point and drag it somewhere around the middle of the diagram. Try moving it up and down. If you move the point straight up, the curve will bow like a rainbow, steeply up at left and then tapering off flatter near the right. This is pushing more pixels (the flatter section) up towards the top, ie towards the white; you're brightening the image. If you move the point straight down, the curve will get flatter at the left, but getting steeper near the right. This is pushing more pixels (the flatter section) down towards the bottom, ie towards the black, you're darkening the image. You can set your first point wherever you want to create your base curve for the image. If you choose to place your point so that the input value is in the exact center of the graph horizontally, you've just set the middle gray; this is exactly what happens by adjusting the gray slider in the Levels control.
  3. "S" Curve?: You should probably be able to tell from the histogram if this is going to work, but I usually grab a point on the lower half of the line and drag it down a bit to form the classic "S" curve, if only because of habit. If it works, great. If it doesn't, simply remove the point. Adding points was easy, each time you click on the graph you get a new point to position. In order to remove a point, grab it, and drag the point off the graph. As the point gets closer to the edge, the curve will deform more and more until it snaps back.
  4. Maximizing Contrast: Now you've set the basic parameters of your curve and it's time to get down to work. You've set your white and black clipping (if any), and set your base shape by adding a primary point to control the overall brightness of the image and possibly a second "S" curve point as well. Now that the preliminaries are over it's time to evaluate your histogram. To start, you'll want to make the curve as steep as possible across the range of values where the peaks are highest. This will create other areas that are flat. Try to contain them to areas where there aren't many pixels (low peaks / valleys). Maximizing contrast may not always work, but it's usually the best way to improve the look of your image and the best place to start. Experiment with different slopes to see what curv e works best for your image. When you're done setting this more advanced shape, you have a decision to make. Based on the importance of the image and how good it looks now, you may want to focus on improving the look of specific areas within the image. In that case you'll need to do some sampling...
  5. Sampling: If you have positioned your image smartly, you can drag the Curves dialog box somewhere that you can see the important parts of you image. When you move your mouse over the image, it becomes an eyedropper. If you click on the image you will see the light value of that pixel appear as a circle on the curve. The Curves adjustment tells you about pixels according to their light value. However, you are probably much more interested in getting an image (or different sections of it) to do what you want them to. Drag the eyedropper around an area you're interested in and watch the circle move along the curve. This will show you the area of the curve that controls that particular patch of the image.
  6. Breakpoints: Once you've sampled an area, you'll know basically the area of the curve that controls the area of the image you're most interested in. Set one point at the beginning and one point at the end of that part of the curve - although at least one of those points may be one of the the two endpoints or the midpoint you used to set the brightness. These two breakpoints will help to isolate the rest of the curve from the changes you want to make. Then drop a point in between and use it to adjust the section of the image that you're trying to control. Rarely you may need a second point on the same interval.
  7. Repeat?: Now take a look at your image. You should now have shaped up the section of the image you were most interested in, however you've also probably shaped up other parts of the image as well. Is there anything else that can be improved? Depending on how fast you need to work and how important this image is, there may not be anything else you want to sample (you might not even sample once) or there may be several. If there are other sections of the image you want to improve, repeat the previous two steps; sample the area, set breakpoints on the curve and adjust. If you are sampling more than a couple sections of the image, the chances become high that the portions of the image you want to affect will overlap on the curve. It's possible that you may get lucky and be able to control both areas of the image with one curve (or you may decide to force it, depending on how close and how important the adjustment is). However, it's more likely the case that you'll have to do some masking and apply a second Curves layer to pull it off. If you're trying to control an especially challenging image, you may need more than two Curves layers.
  8. Casting?: If your image is taken in good light, it will probably look great, and rarely do you need to apply a color cast to an image. However, it's not uncommon that your light isn't good or that there are other extenuating circumstances, and you many need to remove a color cast from a picture. This is where you would otherwise have turned to the Color Balance adjustment, but you'll get much better results using the color channels of the Curves control. The red channel let's you control red-cyan balancing; use it following the same steps you used for the RGB channel (ie clipping and brightness, sampling and breakpoints). When you moved the curve up and down on the RGB channel the image got lighter or darker. When you move the curve up and down on the red channel, the image gets more or less red. If you move the curve up, it becomes more red, if you move it down it loses red becoming more green and blue (cyan). You can also move the green curve up to add more green or move it down to lose green becoming more red and blue (magenta). Finally, you can move the blue curve up to add more blue or move it down to lose blue becoming more red and green (yellow).
  9. Masking: It may be the case that you don't want to apply the effects of a Curves adjustment to all areas of an image. In this case, your last step will be to paint the layer mask white to show the Curves adjustment for the areas you want it to apply, and to paint it black to hide the areas you don't. Also, the chances are high that you'll wind up using at least one more Curves layer to manage the rest of the image, in which case repeat this process again for the other areas of the image.

There will be times when the above advice fails miserably. These cases are rare, but they're not difficult to detect; when you've finished doing the things you normally do, you'll look at the results and say "Yuck." For example if you have a very dark or a very light picture, the "S" curve tends to fail miserably. With all the pixels bunched up at one (or both) ends, the flattened sections of the "S" curve destroy the contrast of the picture. Unfortunately, these pictures (night and low light on the dark end, daytime sky pictures on the light end) typically look obviously unnatural if you shift the steep portions of the curve to coincide with the maximum number of pixels. You simply have to play with these pictures by ear and experiment to see what looks best to you, possibly breaking some of these rules in the process - but you've gotta do what you've gotta do. ;)

Notes

It's hard to understate the importance of the Curves adjustment in Photoshop since CS3. Of the four primary adjustment layers you can apply, one of them, Curves, now does the job of all of the other three. We've looked at how you can specifically eliminate the need to ever touch the Levels, Color Balance, and Brightness / Contrast adjustments, usually with better results because you enjoy much better control. In fact, you can see how each of these three controls was simply a way of automating what the Curves control does in a way that is easier to understand. While it may be easier to understand each of these controls, I think you can see how Curves produces superior results. However, I have heard people argue that Curves is not as effective as using multiple adjustment layers; if one Curves is good, many layers must be better, right? In some cases yes, but here are my recommendations.

  1. If you can accomplish the same job in two ways, use the one that's easiest and fastest. Curves is a difficult tool to understand at first, no question. However, like the Pen tool in Illustrator, it does so much and you use it so frequently that you develop great expertise and speed with it because you get so much practice using it. That usually translates into better speed and results using Curves than a bunch of other tools that you won't use as much and therefore, never master as well.
  2. Curves is much more flexible, and it's easy to use. You have much finer control over Curves and can implement much more sophistacated effects manipulating Curves than you can playing with sliders in other adjustments. This will frequently translate into Curves doing a much better job than more specific tools (e.g. you frequently get much better results controlling brightness and contrast using Curves instead of Brightness / Contrast).
  3. In those cases where multiple adjustment layers would do a better job than one, you should remember that you can use multiple Curves layers.

Usually I will start with my standard Curves workflow and then assess if I can benefit from multiple adjustment layers. If so I'll apply whichever ones do the best job for what I want to effect, like using an additional Curves layer to control the color balance or contrast, etc. If a different adjustment is more appropriate, then absolutely use it, but I find these are the exception rather than the rule. While there are some interesting secondary adjustments you can use, Hue / Saturation is the only one I use regularly. I'm a sucker for color, so I like to increase the saturation on most images to make them more vibrant.