Marnie, speak! Good girl.

Curves

Before we talk about colors, let's learn the basics of how Curves work.

Look at this image of Thea and me at Puppy Agility. I've chosen this image because it has quite a few colors in it already, and the effects of playing with the color, should be more noticeable.

Original Image

To access the Curves, go to the IMAGE menu, to ADJUST and choose CURVES.

You can also use the quick key, [CMND+m] or [CTRL +m], depending on your computer platform.

No matter what your image looks like, your curves dialog box will look the same. You will see a gridded square with a straight line running diagonally through it (that's the "curve" though it's straight right now). The default view allows you to change all the colors in the image, uniformly. We are working in RGB, so we see that the pull down at the top of the Curves dialog box says, RGB. It is the item, in the image below, marked "Channel Selector"

Note the double arrows on the grayscale bar below the grid. Click that to reverse the tonal ranges of the grid. You want yours to match mine. So clicky clicky clicky and watch how those grayscale bars flip back and forth. Weeee! Fun for the whole family.

Ahem

There are two points on the diagonal line already. One is at the top right and the other at the bottom left. These are the highlight and shadow points, respectively. If you do not touch those points, your highlights and shadows will not be effected. Keep those there if you feel that your image doesn't need much of a contrast change.

Lighten or Darken an image

Click somewhere around the middle of the line. A point appears. You can now drag that spot up or down by clicking, holding and dragging. Watch what it does to the image.

Original Image

Dragging the point upwards lightens the whole image without losing your shadows. Note that my hair, pants and the shadow on the tunnel are all still nice and dark, yet the overall image is much lighter. I find that many images benefit from being a little brighter.	Conversely, dragging the point down darkens the images, without losing the highlights. This image shows just that. Thea and my shirt still look white, not gray, but the overall image is much darker.

Changing the contrast of your image

Maybe your image isn't just too dark or too light, maybe you have too much or too little contrast. The rule for curves goes as follows:

The steeper the angle the greater the contrast.

With that in mind, let's look at a couple of examples.

Reducing the contrast of an image

Original Image

To make an image darker and less contrasty, move the top point down. The angle of the line becomes less steep and by moving the top line down, you are darkening the overall image.	To lighten the image, move the bottom point up.The line becomes less steep and the overall image is lightened. Doing this produces a highkey image and gives an etherial effect.

Increasing the contrast in an image

As I stated above, you adjust the contrast by manipulating the curve to be more or less shallow. To increase contrast, you'll want to make the angle of the curve steeper.

Original Image

To make the image darker and increase the contrast, drag the bottom point to the right. The lights, my shirt and Thea all remain bright white, but the image has much more contrast and appears darker overall.	Drag the top point to the left and the image becomes much brighter without losing the deep shadows in my pants, Thea's spots, and on the tunnel.

Adjusting color in an image

We have been working with with all the colors in the image, at once, in the above examples. But there will be times when you only want to adjust certain colors. I use the following technique when photographs of my yarn do not properly approximate the real color.

To adjust specific colors, you'll need to switch from the RGB area to the individual channels. To do so, click the word RGB, above the grid. Select the channel you'd like to modify.

From here, all the techniques we've covered above will still apply. The darkest parts of the image are changed by moving points at the bottom and left of the image. You can adjust lighter colors by modifying the top right corner of the curve. The effects will produce more contrast where the lines are steeper and less where they are shallower.

We'll go through each channel in order, starting with Red.

Red and Cyan

Red and Cyan

In the red channel, pulling a center point up, will increase the red throughout the image. If your purples look too blue, or your reds aren't popping, you can increase the red in the red channel	If want to reduce a red cast, do the opposite. This is very effective in making greenery look greener, and is also effective if your blues look too purple.

Green and Magenta

Green and Magenta

Pulling up and left on the green curve will increase the greens of the image. Paired with reducing the reds (shown above) can improve the look of greenery.	Pulling out the green from the image will increase the magenta. Oranges, reds, and purples may all be improved with this modification.

Blue and Yellow

Blue and Yellow

Increasing the blue in an image, when done delicately, can reduce the effect of indoor lighting. Overdoing it, as we have in this example, can give a slightly clinical effect, but in small quantities is can be quite nice. This is also an excellent tweak for taking the yellow out of a white dog's coat.	Add more warmth to an image by reducing the amount of blue, thereby adding yellow. This will also really improve orange, yellow and red shades.

Conclusion

All the effects I've shown above can be done with a multitude of tools, available in Photoshop. There are specific Adjustment options for Contrast and Brightness, Hue and Saturation, and Color Balance. If those tools work better for you, use them. I personally like to use Curves because you can do so much in a single tool. The more you play with your images, the better you will get at detecting contrast and color issues. Most of us can visually detect a problem, even if we cannot verbalize what is wrong, so using something like the Curves palette, gives you the chance to play around with all of these changes and see what works best.

The above examples are very extreme shifts to give you a better understanding of how the changes effect the image. Make very slight changes, bit by bit, until you hit that sweet spot.

Curves can do so much more, but I'd need to write a whole week's worth of posts to cover even the small subset of knowledge that I have. If you do have any specific questions or if you want to add your own suggestions, please leave a comment.

In the next Photoshop tutorial, I'll be talking about Histograms and Levels.