Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 64

Base Curve
The base curve tab includes brightness and contrast settings. It imitates the
functionality of Nikon's tone curves.
Various camera manufacturers and other sources offer tonality curves on their
websites. To load a curve that you've downloaded or created on your computer,
click the folder icon above the curve editor. Some ready-made curves for Nikon
and Canon are available on Udi Fuchs's website at http://ufraw.sourceforge.
net/Colors.html. You can find curves from other manufacturers at http://www.
dl-c.com/Temp/downloads/download_content.html. And at http://ufraw.
sourceforge.net/Contribute.html, you will find links to more ready-made curves.
For Nikon NEF files, you can choose Custom curve from the drop-down menu
above the curve editor if you want to use the curve that is embedded in the
RAW file. Choosing Camera curve will enable the embedded curve only if it
was enabled in camera.
Figure 2.8
This backlit photo is brightened with the help of the base curve function. Simultaneously, the light areas are darkened. The overexposed
sky gains in color and contrast.