Tone Curves

This often misunderstood control is as much about brightness as contrast

I recently read one popular author's introduction to his chapter on the Lightroom Tone Curve, and he started out by saying that with the improvements in the 2012 Process Version, the Tone Curve was basically outdated. Nothing could be further from the truth. It's true the Lightroom 4/ACR 7 update represented a complete overhaul of the Basic tools that gave us much more powerful Highlight and Shadow correction. That update also eliminated at least one control (Brightness) and reconfigured everything else in the Basic panel to be more symmetrical, starting from a zero position in the middle and having the ability to move them in either direction.

The 2012 Process Version also brought significant changes in the way the Adobe Highlight Recovery works. Three different tools in the Basic panel can now affect, and give you very precise control over, your highlight tones: the Highlights control (similar to the old Recovery control, only stronger), the Exposure control and the new Whites control. I wrote in detail about these new behaviors in the February 2013 issue of DPP.

Wrapping your head around how all the controls work does require some study, testing and exploration. But no matter how sophisticated the new controls have become, the bottom line is that the much misunderstood Tone Curve still provides a very different—and crucial—type of control over your tonal correction. I say that it's misunderstood because beginners seem to assume that the Tone Curve only has to do with the brightness of various tones and don't yet understand that it's as much about contrast as it is about brightness. With the Tone Curve, the two go hand in hand.

So, let's jump in. The Tone Curve panel in Lightroom and ACR is a graph that controls a simple transfer function. Across the bottom of the graph, you have input values (representing tones in your picture, from 0 to 100%), and on the vertical axis you have output values. The shape of the line defines the transfer from input to output. At the defaults, the curve always starts as a straight line from the lower-left corner, up to the opposite corner, which represents no change.

When the line is straight, an input value of 50% (middle gray) will have an output value of exactly 50%, too. Roll your mouse right over the middle of the graph on the line, and you'll see two numbers appear in the upper-left corner. If you're able to get your mouse exactly over the center of the graph, the values will read 50/50. Click down right there, and drag straight up. (You can only click and drag directly on the Parametric Tone Curve in Lightroom. ACR's implementation of the Parametric Curve doesn't allow clicking directly on the graph, only on the sliders. Also, in ACR, you only see input/output values in the Point Curve.)

Figure 1: Default Tone Curve. On the Lightroom Parametric Curve, you'll hit a constraint at 70% or 73%, depending on whether you were clicking on the Lights or Darks side of the 50% mark. Lightroom creates a nice curve for the transfer function, and the input/output values are telling you that this curve is taking an input value of 50% and is increasing it to 70%. Read More...

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