Mastering Contrast and the Tone Curve

Mastering Contrast and the Tone Curve

DxO PhotoLab 7

The Contrast and Tone Curve controls in DxO PhotoLab 7 are subtly different from their equivalents in Adobe Lightroom.


Mastering Contrast and the Tone Curve

One of the questions that people have asked of me is to explain the difference between the Contrast slider and the Tone Curve and offer some guidance as to when to use one, the other or both.

Contrast and the S Curve

The short answer is that the Contrast Tool models an S Curve and depending on where you put the slider, at 100 creates deep blacks, some grey and stark whites, at -100 reduces the blacks and whites in favour of grey.

This slider works on a continuum between the “flat” result at -100 and the high contrast result at +100. There is nothing more to it.

The Microcontrast and Fine Contrast tools can be ignored for this article, they define edges with varying degrees of punch.

Contrast and the Tone Curve

The Tone Curve is a much more complex tool and this is why it takes longer to learn. You can reproduce the effect of the Contract slider by creating an shallow S Curve, you can take that curve and make it brighter or darker by manipulating the Gamma setting while maintaining to a degree, the contrast. between darkest darks and whitest whites.

More than this, you can make multiple points on the curve to tweak narrow ranges of tone in the picture.

This behaviour can be graphically demonstrated in a gradient such as the one in the headline image, but can appear unpredictable in the wild! In fact it is predictable, the secret is to know what the tool is doing.

The Tone Curve - DxO PhotoLab 7

Taking the image here, the first point on the bottom left of the curve represents pure black. The opposite point at top right represents pure white.

The intersections in the background, working left to right represent Darks, Mid Tones and Lights. By placing a point on or near the first intersection and pulling it vertically down, I am making my Darks, darker.

By placing a point on the intersection the right hand side of the curve and pushing it vertically up the scale, I’m making my Lights, lighter.

The Mid tones can be adjusted by pulling the middle of the curve, up or down. This will make the adjustment leaving the two points already defined in the same position.

Gamma Reduction in DxO PhotoLab 7
Gamma Reduction

If I change the Gamma Value, seen below the graph from its default setting of 1, you’ll notice that my mid point stays on the vertical line, as do the Dark and Light points, but they all move, creating a brighter or darker rendering of the image while maintaining the relative darkness and lightness of the Dark and Light points.

You don’t have to stick to just three points, you can have as many as you like, but the more you have, the narrower the tonal range will be for each point. This becomes a subjective decision.

RGB Curves

As well as the Master Curve, there are three Colour Curves, Red, Green and Blue. Manipulating these curves will boost or reduce the value of the Red, Green or blue pixels in the tonal range that you place the point.

Its important to remember with these curves that reducing the value, will effectively boost the opposite colour on the colour wheel.

  • Red/Cyan
  • Green/Magenta
  • Blue/Yellow

These curves can be combined to produce any colour you choose, as you might expect since we deal exclusively with RGB values in digital photography.

Colour Curves Combined DxO PhotoLab 7
Colour Curves Combined

In this image I have created an orange cast in the Darks by boosting the red curve and reducing the blue (boosting yellow) You can see that the effect fades in line with the curve as we move into the mids and Lights.

Restricting the Tonal Range Affected
Restricting the Tonal Range

Inn this image, I have created a sharp boundary between the darks and the mids by manipulating the red and blue curves.

A Workflow for Tone

When you edit a RAW file, you can use the contrast slider to do what I call a “ballpark” adjustment. Assuming you want more contrast, nudge the slider to the right until you feel that it is close to what you envisage in the final picture.

Then, use the Master curve in the Tone Curve tool to finesse the contrast you have created with the Contrast slider. As you get more comfortable with the Tone Curve, you may find it’s the only tool you need, but it does require experience to master.

Finally, if you need to adjust the tonal range you can use the colour curves to enhance the colours in various ranges of tone.

The DxO PhotoLab 7 Video Series

There is a series of videos, fourteen to date, covering various aspects of PhotoLab 7 from an introduction to the interface to more advanced topics like DxO – Control Lines and Grad Filters.

The Whole Series is listed here, DxO PhotoLab 7 Videos and contains links to every video completed.

Alternatively, the DxO playlist containing these videos can be found on YouTube.

For those who prefer text and photos to video, some of the series are available as articles.

DxO vs Adobe Repair and Clone Tools

Creating a Signature in Photoshop

DxO – Control Lines and Grad Filters

Get the Tools

You can get a trial version of DxO PhotoLab here.

You can buy DxO PhotoLab 7 here.


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