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and he started out by saying that with the improvements in the 2012 Process Version, the Tone Curve was basically outdated.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.)
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%.
The Tone Curve is a simple input/output (Transfer) function. Okay, that’s more or less a brightness adjustment, right? But to get the most out of the Tone Curve, you have to understand what this is doing to your contrast at the same time. I’ve found the best way to see how that works is to use a grayscale. This simple graphic has 11 values that step from pure black to pure white in 10% increments. It was created in Photoshop and gives you a very graphic view of how the Tone Curve is pushing your tones around. With a Default Curve (a straight line), the tones in this grayscale are nice and evenly spaced out on the histogram.
Grayscale with Default Tone Curve. An increase in contrast generally means that light tones are going to get lighter, and dark tones are going to get darker, right? Let’s see what that looks like. Push the Lights control up to +60, and pull the Darks control down to -60, creating a classic "S curve." With those two adjustments on the curve, you’re indeed pushing tones away from the middle, creating greater tonal separation in the middle tones. Greater tonal separation equals greater contrast, right? But, then, look at where the tones on the extreme ends are going. In the very darkest and lightest parts of the image, out at the ends of the histogram, you aren’t seeing tonal separation, but rather tonal compression. The tones at the ends of the histogram are being compressed together, creating less contrast.
A classic "S curve" increases contrast through the midtones, and that’s probably the most interesting thing about the Tone Curve. When you add contrast in one part of the histogram, you have to give it up somewhere else. This is easy to see in the grayscale, but with a little practice, you’ll be able to see it right in the shape of the curve you’re creating. Anywhere that you’re creating a portion of the curve that’s greater than 45 degrees—as you clearly see across the midtones in this example—you’re increasing contrast. And anywhere the curve is less than 45 degrees, you’re flattening, compressing tones or reducing contrast.
Figures 5, 5-a & 5-b:
A classic "S curve" increases contrast through the midtones. Okay, how do you translate that to an image you’re trying to correct? Let’s look at the photo of the old woman wearing a scarf in Figure 6. This is a scan from a black-and-white film negative, and I feel that the basic black and white points are good.
Old woman portrait with Default Curve, but without any further correction. The scan just feels a bit flat and dark. So, I know I need to brighten up my midtones and add contrast. Pushing up the Darks slider does indeed brighten all the midtones, but the curve that’s created also has a giant flat spot right across the middle, flattening the contrast in those areas.
Figures 6-2a & 6-2b:
Old woman portrait with +60 Darks Curve. On the other hand, an almost identical move on the Lights control also brightens the midtones. This curve creates a steeper line through the midtones, increasing contrast in the vital skin-tone areas, creating a much better correction.
Figures 6-3a & 6-3b:
Old woman portrait with +40 Lights curve. For the photo of the men pulling the cart, I also want to brighten up my midtones. But this time I want to do it with a reduction of contrast. In this case, trying the Lights control just drives the contrast up out of control. The look I’m going for this time is to re-create the very hot and kind of washed-out sunlight that I remember when this photograph was taken. Pushing up Darks to lighten the midtones give
s me softer and more open shadows, which is the mood I was looking for.
Men pulling cart with Flat Curve. Very roughly speaking, increasing Lights will increase contrast across your midtones, where increasing Darks will decrease contrast in the midtones. Both of these moves do lighten the midtones, just in very different ways. And the reverse is also true. Decreasing Lights will decrease midtone contrast, while decreasing Darks will increase contrast in the midtones. Having said that, 99% of the time I’m pushing either the Lights or Darks up in tone, with the Shadows control being great for crushing blacks down, giving you control over the contrast in the very darkest parts of the image.
Men pulling cart with +50 Darks softens the midtone contrast. Lightroom and ACR both also have a Point Curve panel that works in a similar way as the Parametric Curve. There are two big differences with the Point Curve. First, you can put as many points on the Point Curve as you like, and there are no constraints. You can put them wherever you want, and drag them anywhere you like. So, be careful! The other big difference is that, starting with the 2012 Process Version, the Point Curve now has R, G and B curves, in addition to the Composite Curve. Having individual Red, Green and Blue curves gives longtime Photoshop users the control they have become accustomed to, with the ability to fine-tune color shifts, as well as white and black points in individual channels.
George Jardine is an authority on Lightroom, a program he helped create when he was at Adobe. His excellent tutorials are available at www.mulita.com.