Level: Increasing Basic to Advanced
The curves tool in Photoshop is one of my favorites simply because it is so incredibly powerful. Brighten and darken, work on contrast, color balance – nothing this little tool box cannot handle. The curves tool looks complicated but actually it is pretty simple to use. Just a bit of practice might be needed. Let’s have a deeper look into what it offers.
Brightness Change with Curves Tool
Changing the brightness of an image with the curves tool is kind of the very basic usage but even in this simple task this tool shows to be extremely precise and powerful.
Open an image in Photoshop. In the adjustment tools section click on the curves tool. A pop-up slider will open, showing something like the featured image of this post.
The diagonal line is what we are going to work with. If we push the line up a little bit resulting in a bow we brighten our image, if pushing down we darken the image. How simple is that! Click on the eye icon in the bottom of the curves tool pop-up window and check the changes, on, off, on, off.
Ok, next step, we now want to brighten just the darker parts of the image. Click on the little backwards arrow in the bottom of the curves tool window to reset the line to the diagonal default. Click the middle point of the line once without moving it. We should now have created an anchor point. Grab the middle point of the lower half of the diagonal and push it up slightly. Click the eye for checking the effect. Only the shadows should have brightened up.
Doing the same with the upper part of the diagonal you would have brightened the brighter parts of the photo only. In case you don’t see anything the eye icon might still be in “off” mode from your testing before.
Don’t tell me the curves tool is difficult. We choose whether to brighten the whole image by simply pulling up the whole diagonal line. Or we opt for changing the brightness of just parts of the image by setting anchor points for the tonal range we want to change and then move the line in between those anchor points up and down for brighter and darker.
But this curve tool is capable of much more!
Excursion: The theory behind, reading the curves tool, is not necessary to understand but for some of you it might be helpful. If you simply want to work on your pictures don’t care about this excursion.
The bottom line shows the before (input) and the vertical line on the left shows the after (output). Choose any brightness tone in your image on the gradient in the bottom line. Then with your finger move straight up to the line and go left until you hit the vertical gradient. This value now shows how bright your chosen point from the bottom line will show up afterwards.
In case your line is untouched it will be a centered diagonal from one corner to the other. Well, in this case all the input values from the bottom gradient with this finger technique will show up exactly the same values in the output gradient on the left.
If we now push up the line a little bit, you will discover (with the finger technique again) that each value from the bottom line will result in a brighter value on the output line on the left. We have a brighter image.
One more topic for background: The mountain-scape line behind the graph is called a histogram. It shows the distribution of dark and bright pixels in your image. The column furthest to the left shows how many totally black pixels you have. Moving right a bit the column shows how many dark pixels of a specific brightness can be found in the image, in the middle you have the 50% greys seen from a brightness perspective, on the right we see how many brighter pixels we have and in the very right it’s the blown out whites.
Therefore usually a column on the extreme right or extreme left of the histogram should make you have a second look on your image. It would indicate a big part of the image either be black or totally blown out. Sometimes that’s the effect we wanted to achieve, but in general we look for an even distribution. Enough background, back to using the curves tool practically.
Little Helpers in the Curves Tool
On the left of the curve graph you’ll see a couple of tools, some little helpers. Click the hand icon and move the cursor across your image. Wherever you go it now shows the brightness of the pixels on your line. With a click you create a point on the adjustment curve. If you click, hold, and drag up and down you can work in the image itself without touching the curve. I rarely use this helper.
The three sample tools enable you to set a black point in the image (the darkest point you want to be pure black), a grey point for 50% Grey helpful in color correction, and a white point for the area you want to be the brightest and show pure white.
The curves line is the default option we worked in before and the pencil for drawing a curve manually you will most probably not need. The last icon automatically compresses your curve step by step back to being a line. No need for that in my eyes.
By the way, your anchor points you can simply drag and drop out of the window to delete them. Alternatively click on the point for activating it and hit the Delete-key.
Contrast Change with Curves Tool
Since we know how to brighten or darken parts of the image it is not really far to changing the contrast in a photo. Reset the diagonal with the backwards arrow in the bottom first. Push the upper part slightly up (bright tones will be a brighter) and then pull the lower part of the line not only back to where you started but even beyond (dark tones will be even darker). Result is an S-shaped curve. Check with the eye icon: the contrast in the image increased.
Tone Clipping with Curves Tool
Sometimes our images are a bit flat even beyond lack of contrast. No white point, no real blacks, just dull in between. Curves tool rules, we learned, so let’s challenge it with this task.
Use the sliders below the gradient in the bottom (little triangles) and move the left one a bit to the right and the right one to the left. The effect behind is clipping some tones, more or less cropping away some tones in the very end. This effect will only work well on flat images!
Whenever using this technique make sure not to go too far. At best hold the Alt-key while moving the sliders. You will see an indicator when blowing out the highlights or loosing detail in the very dark parts of the photo.
Color balance with Curves Tool
As default the curves tool works on all color channels together (Red, Green, Blue: RGB). See the dropdown menu RGB right above the graph? Change it from RGB to Red, which means we will now work in the red channel only. Pushing up the curve a little bit will push it to the red side, pulling it down the image will be more cyan. If you want to change the color balance in the highlights only we just move the upper part of the curve and anchor it in the middle point.
Same you can do with the other channels. The pairs are: red-cyan, green-magenta, blue-yellow. Working in the color channels will need some practice for sure. But your options of control are near to unlimited.
The Curves Tool in Lab Mode
The Lab mode is very special and when working on the colors of an image I usually combine switching to this mode with working in the curves tool
Other than in RGB mode in Lab mode the color channels are separated from the brightness channel. L is the channel for Lightness, a and b are the channels for the colors. All of it is just a different algorithm of putting your image together, not using a wild mixture of red, green, and blue but light, a, and b instead. It’s a bit more abstract.
Using it is simple again. Create a copy of the image you are working in (Image -> Duplicate). It opens a new tab with a copy of your image. Go to Image -> Mode and choose “Lab color”. It might tell you to lose all layers. Click ok. Since we are working in a copy of our image we still have the layers in the other tab. Photoshop now transforms the algorithm behind the image to the new mode. You don’t see any effect. It’s just a different math with the same result. So what, I made you work for nothing? Wait wait, patience.
Change Brightness with Curves in Lab Mode
Open a curves layer. Here the differences start to show up. Instead of having RGB in the dropdown channels menu now we see Lightness as the default channel. Moving the curve up and down will still change the brightness of your image. But the big advantage of working in Lab Color Mode is that our light channel is separated from the color channels. That means a shift in brightness here will have no effect on the color saturation, which unfortunately always is a side effect when working in RGB mode.
Change Color Saturation with Curves in Lab Mode
For working on the colors of the image we have to leave the Lightness channel behind. In the menu above the graph, switch from Lightness to “a”. We are now working in the a color channel (sounds weird and abstract, just take it as it is). Move the end points of the diagonal line inwards (with the little black and white sliders in the bottom) exactly the same amount, for example to the first line of the grid from the left and right. Result is a color shift. Looks awful, I agree. But if you switch from a to b channel now in the dropdown and do the exactly same move on the sliders here (same amount as in a channel), things are balanced again.
What happened? We increased the color saturation of the photo. Usually the effect is way too strong, we will have to adjust a bit for example by lowering the layer opacity in the end. But that we do after re-importing the result to the file we started working with before creating this duplicate.
Tipp: I usually work with a finer grid in the graph (in the upper right corner of the curves pop-up window you can enter the settings. In the menu point “Curves Display Options” you can switch to a finer grid view.
Change Color Balance with Curves in Lab Mode
Even better. If we want to work on colors, giving them a strong shift, then working in Lab Color mode is really pleasant. Here you will have to experiment on your own. But moving the curve up and down, left and right, moving the end points in either direction in each color channel a and b will very quickly give you a very good feeling what is going on. You will wonder about the quality of the color changes!
Re-Import the Lab Changes
Since we are working in a copy of our image for the Lab Color mode modifications we have to re-import our copy to the main photo we were working in. First flatten the layers in the Lab copy (right-click on the Layers -> Flatten Image). Then simply drag and drop the only remaining layer into the window of the previous image we worked in. It will be added as a layer on top of all the others.
If you want the effects just applied in something specific areas of the photo like the colorful rain coat of a person or the color of a car you can additionally work with a layer mask to ensure changing this part of the image only. The perfect masking for this kind of task will be topic of my next masking tutorial.