December 29 2016
The Tone Curve panel in Adobe Lightroom provides another level of adjustment for brightness and contrast in addition to the Basic Panel tools.
The Tone Curve offers more targeted adjustments than the general purpose adjusters in the Basic Panel. There is a degree of interaction between the two a sets of adjustments and the histogram is an essential tool in coordinating the two. There is no right way to use the Tone Curve in conjunction with the Basic Panel tone sliders.
The Tone Curve is similar to the powerful Curves tool in Adobe Photoshop. The default state is a much simplified version of Curves, offering fast easy adjustments, particularly for photographers new to advanced photo post processing. The Tone Curve uses four sliders to provide quick adjustments of brightness complementing the adjustments available in the Basic panel. The Tone Curve has sliders to adjust the Highlights, Lights, Shadows and Dark sections of the image.
Confusingly, even though they have the same names the tone adjuster sliders in the Basic Panel are different from their namesakes in the Tone Curve Panel.
The Basic Panel’s Highlights and Shadows sliders control a complex analysis and adjustment algorithm that Adobe keep improving. The Tone Curve adjustment is a simple curves adjustment where the exposure level in the target region is manually adjusted. The Basic Panel adjusters control the amount of processing applied to the larger target area not a direct control of the exposure level. Because of the different scope and methodology of the two panels they complement each other. It is not a case of one is better than the other, it is usual to use both.
The essence of the curve tool is a graphical representation of light levels as a straight line from black on the left to white on the right. The height indicates the amount of brightness along the line on the graph. Four slider adjusters raise or lower a segment of the curve, adjusting the brightness in that segment. The shape of the curve determines where there are contrast changes. A steeper curve results in more contrast.
It is also possible to adjust the positions of the transition points between the adjustment regions.
There is a histogram background on the Tone Curve display showing the number of pixels at each brightness level. This is one of Lightroom’s indications of where on the curve to apply the adjustments. Another is pointing to a spot on the Tone Curve to highlight the adjustment slider for that region.
Reset adjustments in a the panel using Option key (ALT key in PC)
Another useful indicator is the targeted adjustment tool. Located on the top left corner of the Tone Curve panel, the circular icon toggles this tool on and off. Point to a spot on the photograph in the preview window and this tool shows the corresponding spot the Tone Curve. If users click and drag the tool up and down on the image, the adjustment is centred on that region on the Tone Curve.
On custom tone curves it moves an existing adjustment point up or down. If there is not one for that part of the tone curve it adds a new point.
This flexibility and the ability to target specific tonal regions is one area where the Tone Curve differs from the Basic Panel.
Clicking on the curve icon in the bottom right corner of the Tone Curve panel changes it to a custom curve tool. This is a standard curves tool where users add and move anchor points on the curve for more control over the shape of the adjustment curve. This provides even more specific targeting of the adjustment point on the curve. The adjustment curve is superimposed over a histogram to indicate the points on the curve to adjust for a particular image.
Click on the control icon in the bottom right corner of the panel to activate this mode. Now users can add extra control points on the curve and have full control over the amount of adjustment, crazy amounts are now possible. Point and click on a spot on the curve to add another control point.
Point and drag to move a control point along the curve. Flatten the curve with a right click of the mouse, if it all gets too messy. Hovering over a control point enables the removal of that point, or flattening the curve.
In the Point Curve mode users can create and save their own tone curves. The default is the linear curve without any adjustments applied to the image. There are two other options in the Point Curve dialog, located below the curve graphic, for preset medium and strong contrast adjustment curves. These can be used as they are, or as a starting point for a new custom curve. Users save their new curve as a custom preset using the Point Curve dialog. This is a contextual dialog that adds the save option when a custom curve is created.
If you click the curves icon the Tone Curve goes back to the standard slider view and the curve is now based on your custom curve.
In the Point curve mode you can select a colour channel to adjust as well as the composite RGB channel.
There is still the normal Lightroom histogram panel at the top of the Develop module. This is more than the normal display of the number of pixels for the range of tones from black to white. There are clipping indicators in the top corners to show when clipping occurs.
The colour of the triangle corresponds to the colour channel that is clipping. For example, if the little triangle is yellow then yellows are being clipped, but red and blue are not. If the triangle is white then serious clipping is occurring on more than one colour channel.
The other aspect is showing where the clipping occurs in the preview image. To enable this feature click on the histogram’s clipping triangle (or keyboard shortcut “j”). White areas that are clipping are shown as red and black areas are blue. The it is a creative decision about the importance of those areas to the photograph as a whole. Some areas of a photograph are not important. It may be better if these are clipped if it means the exposure for the main subject is optimised.
Everything interacts and adjusting one area of the tone curve affects other areas. The Lightroom systems are designed for fast small adjustments, for complex solutions the next step is Adobe Photoshop. Because of the number of variables it is usually slower, but offers far greater capabilities for adjusting photographs.
Remember Lightroom is a non-destructive editor, no changes are made to the original image file, they are only applied to any exported files.