A Viewfinder Darkly

Photography tips and tutorials

How to Use GIMP Curves Tool for Selective Exposure Adjustment

March 7 2013

by Philip Northeast

Photographers use the curves tool in the Gimp photo editor for adjusting exposure and contrast in parts of a photo with similar brightness, or tonal levels.

Curves is a form of selective exposure adjustment targeting the tonal range between the black and white points that were set using the levels tool.  Photographers can go straight from the levels tool to the curves tool by a single click in the levels tool. This illustrates the relationship between the two exposure adjustment tools, with curves following levels adjustment.Levels specifies the limits and overall brightness but with curves photographers can adjust the relationship between parts of the tonal relationship.

Curves Tool

The Curves tool from the GIMP

The straight line on the curves graphical representation indicates the relationship between the tone values of the original photo and new tonal values produced by adjusting in the curves tool. The straight line shows relationship between the original photo and the adjusted tonal values is the same. Dragging a point on the straight line higher makes a curve so the tones are brighter and a downward curve makes the tones darker. Because changes result in a curved section there is a smooth transition between areas of different tonal value.

The histogram is used as a background in the curves tool showing the number of pixels in the photo for each tone.

Adjusting the exposure using curves allows photographers to alter brightness and contrast without blowing the delicate highlights or losing details in the shadows.

adjustment points

The curves tool with adjusting points added to the straight line

Clicking on the line in the curves tool creates an adjusting point for increasing or decreasing the tone of the pixels under that part of the line.  The adjusting point does not move when other points move, so they are also anchor points on the line.  Adjacent adjusting handles remain in their position and only the targeted handled moves. This creates a curve in the line, hence the name of the technique.

Practical Application

The curves tool adjusts areas of tonal value, not specific areas of the image.

In the sample photo I have added a number of adjusting handles on the straight line of the graph representing the relationship between the original photo, the horizontal base, and the processed version represented by the vertical axis.

They correspond to:

• shadows

• darks

• mid tone

• light

• highlights

Dark Areas 

Using this approach it is possible to adjust the darker areas of the photo without making the shadow areas too dark. In the example photo we adjusted the exposure of the mountain without changing the exposure of all the intersecting bits of building, and complex rigging of the yachts. The Organ Pipes on the mountain are part of the shadow area, and we want  to show the texture, rather than just a dark silhouette.

The water in the foreground is lighter and is in the region controlled by our dark handle. So we can make the mountain face a bit lighter by raising the shadow handle, and keep the water dark by moving the dark handle down.  This did not affect the mid tones or the lighter areas.

Adjusted curves

The curves tool after adjusting the photograph

This shows the value of setting your own adjusting points to suit the tonal characteristics of a particular digital photo.  Sometimes fewer adjusting points produce better results, it is a matter of matching the position and number of points to the photo.

Light Areas

Now, looking at the white cloud areas. We can brighten the sky by raising the lights handle while the highlights handle anchors them and prevents clipping of the brightest areas of the clouds and losing the fine texture.

For this is photo the mid tones, or overall brightness, remains the same. The curves adjustment helped bring out the fine detail after the contrast adjustment in the levels tool more precisely than the broad overall adjustment of the midpoint slider in levels.

After curves

The Waterfront after adjusting with curves in the GIMP


The initial placement of the adjusting points is not critical as they can be slid along the curve  to maximise the adjustment at a specific point in the range of light tones.

Using the curves tool is only part of the digital workflow for optimising a photograph. It is a specialised tool for use in conjunction with others in achieving the final photograph.



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