Using Curves in Photoshop

If you’ve ever struggled to get your head around curves adjustments in Photoshop, you found the right tutorial. We’re going to look at using different points on the curve, what they represent, and using the different RGB channels to make specific colour adjustments.
This tutorial is part of a PSDFan series on professional retouching, and as a digital retoucher I spend most days using curves in one way or another. Once you have a grasp of the basics, you’ll have at your command a seriously powerful tool for editing your photographs.
First, here’s what your flat, basic curve looks like before adjustments;

I’ve marked the curve to show what each point represents. It’s a scale that represents the starting exposure level of your jpeg or RAW conversion. the bottom left point is the blacks in your image, the top right is the whitest point and a gradual scale of everything inbetween. The middle point represents the ‘midtones’.
If we raise a point on the curve it becomes lighter, or more exposed. Pull a point down and we decrease exposure. Clicking the middle of the curve and pulling the midtones up or down will lighten or darken the whole image, but will keep the black and white points where they are;

If we pull the black point higher, we lose the pure blacks;

And pulling the white point down we lose the pure whites;

You can experiment to add more points, for example you can increase midtone contrast with an S-curve;

We don’t just have to stick to luminosity values, we can use the dropdown menu in the Curves dialog box to affect the individual channels of Red, Green and Blue (RGB).

Be aware that each of these channels has a friend in the CMYK colourspace; decreasing blue will introduce more yellow for example. These relationships are;
So if we remove green, the same thing happens as if we increased the magenta curve in CMYK mode;

These relationships exist in CMYK mode of course, so if you’re working with CMYK curves for print, you can simply decrease Cyan to increase Red.
You might see a lot of this kind of processing in magazines at the moment;

Notice how the black point isn’t completely black? And how there’s a lot of blue in the shadows? I’ll let you experiment with it
This is the third in a PSDFan series of professional retouching tutorials aimed at teaching good practice for advanced photo editing.