Curves is certainly not one of those intuitive features of Photoshop (or Lightroom), or at least that was my perspective when I tried to figure it out on my own before I got formal training. Curves is used to brighten or darken tones in your image, and at a more advanced level, to do color correction work. The best way to use it in PS is with an adjustment layer (Layer>New Adjustment Layer>Curves.) In Lightroom simply go to the Tone Curve panel in the Develop module. The curve will look like this (with different buttons and features around it depending on your software and version). Photoshop gives you more point-by-point control, so I will begin there.
So how do you read the curve? It is a two-dimensional chart showing tones before and after. Along the bottom (X) axis, you have tones ranging from pure black at the left to pure white at the right. These are the tones of your image before you make any changes. Along the left side (Y axis) are the tones, again from pure black to pure white, AFTER your changes to the curve. The diagonal line running bottom left to top right says what change you are making. If you visually run a line straight up from a tone on the x-axis Before to the Curve and then straight over to the left to After, you will see what that tone will be after, as shown below:
Of course in this case we haven’t moved the Curve, so there is no change from before to after, all tones are the same.
Let’s put a point at the center of the curve by clicking on it, then hold and drag straight up. Now if you follow medium grey up from the Before axis to the Curve and over, you see that it is now a significantly lighter grey, so we have lightened that tone. All other tones in your image have been lightened as well, other than pure black and pure white — but not by as much as the midtones. (Note that the histogram has changed in the screen print — it is simply that when I started writing this post I was using one image, than a few months later when I got back to it, I had to choose a different one — so ignore it.)
The opposite would happen if you put a point in the middle and you dragged downward — you would darken all tones in the image except pure black and pure white, but the most darkening would be applied to the midtones.
This technique to brighten or darken your image protects you from blowing out your highlights, or blocking up your shadows. A point in the center moved up or down achieves the same effect as using the Brightness slider in LR or ACR.
You can target the tones you want to affect and protect others — for example, in the following screen print, I brighten the lighter tones, and put two more points on the curve to prevent the midtones and darker tones from brightening as well. Be careful that you don’t make the curve too abrupt.
Hint: If you put a point on the curve that you want to delete, click on it and drag it off the chart.
Adding contrast to your image involves brightening the bright tones and darkening the dark tones. This is done with a classic S-curve:
Of course you can also reduce contrast in an image, by darkening the bright tones and lightening the dark tones:
For contrast, points exactly on 1/4 and 3/4 tones, as I demonstrated above, give you the same result as you would get with the Contrast slider in Lightroom. With the curve though you can make a more conscious choice of which tones to focus on.
Lightroom also allows you to click and drag on the curve to bend it, though visually points aren’t laid down. However, you don’t get quite the same degree of fine control as you do in Photoshop. In a way, this is a good thing — Lightroom protects you from yourself by not allowing extreme (and therefore potentially disastrous) curves with many, many points or shifts in direction. On the other hand, if you know what you’re doing with curves, and want that fine control, you will want to take your images from Lightroom into Photoshop to achieve it.
Lightroom also offers sliders below the tone curve. This is particularly handy for those who don’t understand the curve itself, but is also a convenience for all of us. Highlights, Lights, Darks and Shadows by default each represent 25% of tones from black to white.
If you want to brighten highlights in your image you can click and drag on the curve, or you can drag the Highlights slider to the right. If you want to target just the brightest 10% of tones, for example, rather than the brightest 25%, you can narrow the range that the Highlights slider affects by clicking on the triangle slider at the base of the curve diagram (i.e. on the x-axis) and dragging it to the right to redefine “Highlights”, and then drag the Highlights slider to brighten:
In the same way you can redefine what tones Shadows, Lights and Darks cover.
In Lightroom, I tend to use Brightness and Contrast in the Basics Panel to achieve overall midtone Brightness and Contrast, and then I move to the Tone Curve to finesse any specific tones that need attention — often I use it to dampen down my highlights, or to brighten up deep shadows.
Give it a try!
UPDATE MARCH 2010: The point curve, previously only available in ACR and Photoshop, has now been introduced in Lightroom 3 Beta 2. Click HERE for my post on how to access it.