The Curve dialog in Photoshop hides some surprise features that aren’t immediately apparent at first glance. In Photoshop CS3 and later, one surprise is that you now have the adjusters under the chart that you’re used to using in the Levels dialog inside Curves. So it’s easy to perform a Levels adjustment in Curves – the reverse isn’t true – Levels can’t do a Curves adjustment. So I don’t use Levels for increasing tonal range in my images anymore – Curves does everything I need.
Second tool is the sampler. If you want to add contrast in a particular area of an image it helps if you can find that area on the curve. Simple. Just move across and hold the left mouse button as you wave your mouse pointer over the area of your image to sample. When you do this, you’ll see a marker move along the curve showing you where those pixels are in the curve. This is the area you want to steepen – the steeper the curve, the more contrast in that area. So pull the curve above the area you want to affect upwards and pull the curve below that area downwards to steepen the curve and you’ll get more contrast in that area of the image.
To add markers to the curve – Control + Click (Command + Click on the Mac) on a point in the image and you will add a marker on the curve indicating exactly where those pixels are to be found. You can use the marker to anchor the curve so it doesn’t move when you pull the curve above or below it or drag on the marker to change the shape of the curve at that point.