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Put simply, the curves tool allows you to change the brightness of pixels in your image. When first using the tool you will notice that the curve is not a curve at all (not yet), but a diagonal line running from bottom-left to top-right. This line represents all of the tones in your image - the lightest point at the top-right and darkest at the bottom-left. By adding anchor-points and changing the shape of the curve we can, for example, increase the overall contrast of the image, or adjust colour balance (more on this later).
The following image illustrates how dragging a point of the curve changes its tone.
Common curve styles
Here are a few common curve styles, along with an image to show what effect they have. The first image has had no changes made to the curve.
The following image has had an s-curve applied, further brightening light areas and darkening areas of shadow, increasing the images overall contrast. Subtle adjustments to the curve are usually best to avoid losing image detail.
This image has had an inverted s-curve applied, brightening dark areas and darkening light areas, decreasing the images overall contrast. Again, subtle adjustments to the curve are best to avoid flat-looking images.
By default any changes to the curve are applied to all colour channels (red green and blue). However, individual channels can be tweaked to correct colour balance, or to produce interesting effects. The image below has has an s-curve applied only to the blue channel, increasing blue levels in light areas - most noticeably on the shoulders, but reducing them in dark areas.
Care should be taken when adjusting curves to prevent posterization (unless this is an indended effect). Posterization can occur when too severe a curve is applied - replacing areas of subtle graduation with distinct blocks of colour.
Useful tips for Windows users
- Ctrl + M is the shortcut to the Curves tool.
- Holding down the left mouse button over your image will show you where on the curve the currently highlighted part of the image is.
- Pressing Ctrl + Left mouse button will set an anchor point on the curve, based on the mouse pointers position over the image. Very useful if there is a specific area of the image you want to adjust.
- Holding Alt changes Cancel to Reset in the Curves window, allowing all changes to be discarded without closing the window.
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