2. Tone curve: your single point of success

Who shoots raw does that presumably to have more control over the end result. This implies as well that you stop using auto-buttons and expose the photo ‘by hand’ using the tone curve. That’s really easy, as shown below.

The term ‘single point of failure‘ describes a system, for example a computer network, that is built in such a way that if one component fails, the whole system stops working. In a similar way I think about the tone curve as a ‘single point of success’: here you make or break your photo. It is therefore highly recommended to spend some time on this tool and see what it does.

As you’ll see shortly, the curve does not only influence the shadows and lighter parts of the photo, but also the colors. My statement here is that in 80% of the time, you can produce fine results with only this tool – and with this tool only.

I open the following raw file (a NEF) with the neutral processing profile.

neutral.profile

As a reference I include the JPG that was produced by my camera (I shoot raw+jpg most of the time). Click on one of the photos and you can toggle between them with the mouse or the arrow keys on your keyboard. The histograms are included. As you can clearly see, the jpg is brighter (because in-camera processed) and the histogram more balanced. It simply looks better (for now, that is). You can see as well the effect of my standard exposure compensation setting of -0.3 EV to avoid blown-out highlights: the histogram does not touch the right vertical axis.

It is easy to bring this nef on par with the jpg. In the toolbox, go to the Exposure tab and click in the first entry (also called Exposure) on the arrow in the Tone curve 1 section (indicated in red here)…

exposure.exposure

… and choose Control cage. Keep the drop down menu on the right on Standard. That gives this.

tone.curve1.base

Remember, this is a raw file without any post-processing applied, that’s why the histogram in the tone curve window isn’t well balanced. The first thing I do is to move the upper right node (the black dot) a bit to the left, somewhere to the point where the histogram ‘stops’. If you move the node too far to the left, you will loose details in the highlights. If you activate the ‘Clipped highlight indication‘ (click the icon on top of the application window or hit the keyboard shortcut ‘<‘, less than), you can see when things start to clip.

clip.indic

Please note that RawTherapee’s main histogram – that reflects the effects of the tools we use – changes during this action: it moves to the right. Should we move the node too much to the left side, the main histogram will touch the right axis more and more: clipped!

tone.curve+histo

I repeat this for the left side of the histogram and move the lower left node a bit to the right. I keep an eye on the main histogram and see that the curve moves leftward now. Same story: when you move the node too much to the right, the main histogram will move to the left, leaning more and more to the y-axis, as shown below.

clip.left

So dont’t push things too far. I choose this tone curve for now.

tc1.adapted1

And the raw looks so, including the new histogram. I include again the original camera jpg.

The processed nef and the jpg are closer already.

Next step is to add two points on the curve, one in the deep shadows area and one near the highlights. Drag them a little bit to the right and to the left, respectively, like so. This augments the contrast.

tc1.adapt2

Last step is to add one more node, somewhere in the middle of the curve. Adapt at will until you find the light and the atmosphere you saw when you took the photo, or another atmosphere. Moving this node one centimeter or half an inch to the left or the right, or lower towards the shadows or higher towards the highlights, can have an important impact on the image. Take the time to investigate this. My final tone curve for this photo looks so.

tc1.final

And this is the final image, with again the camera jpg to compare. Of course this is only ‘1 final image’, in reality there are many. Up to you and your imagination, visualization, creatization…

Fine control

As you might have noticed, changing the position of a node on the curve can have a dramatic impact on a photo. There are two ways to have a better control on repositioning a node. One is to press and hold the Ctrl key when moving a point. This slows down the speed of the movement. The other way is to click the edit icon (in red here under) and right click on a node.

fine.tune2

Now you can place the mouse in one of the two fields under the curve and you can change the values there either by 0.1 using the arrow keys, or by 1.0 using the Page Up and Page Down keys. The left field labeled ‘I’ moves the node horizontal, the right field labeled ‘O’ vertical (input and output, I suppose). Right click somewhere in the graph to deselect the node.

If you want to change a certain area, click the cross icon on the right, indicated in red here under.

area.select

You’ll see a vertical bar that moves when you move the mouse over the photo. On an area of interest, hold the Ctrl key and click the mouse to add a new node to the curve. Delete nodes by dragging them outside of the tone curve window. You can save and load curves, or copy them to the clipboard with the respective icons on the lower right. Reset the curve with the undo icon, top right, in red below. You can use as many nodes as you like, but four, five or six will often do (including the two basic nodes).

reset.curve

There’s one rule to respect though: do not exaggerate. When a curve gets ‘too curved’, when the nodes are too far apart from each other, you’ll introduce posterization or other unwanted side effects.

Below are again the four photos we have used here. The raw with a neutral profile, the camera jpg, the raw with the first two nodes moved to the left and the right, and the final version with three extra control points.

 

Part 3. Sharpening


hor.bar
pm|dec15

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