Development / Knee
Actions
- knee/setup : Creates a Curves Adjustment Layer to remove highlight clipping.
The knee action generates a Curves Adjustment Layer which can be used to apply a film like tone curve to compress highlights, and thus avoid highlight clipping. The knee is designed to be applied before any other development.
Here is a crop of an undeveloped image with a large dynamic range. Note how the highlights extend far away from the majority of the tones. This tonal distribution is typical of nearly all large dynamic range photographs.
Below is the result of only applying the knee. See how the tonal range is now reduced and more compact, and the photo appears to have lower contrast.
Here is the same crop without the knee but after development. Notice the highlight clipping which is clear in both the image and the histogram.
And now the same development with the knee layer turned on. Notice how now the highlights all fall in range, and are no longer clipped.
Adjusting the Knee
By default the knee/setup action produces a medium amount of highlight compression. Note that in a linear color space, that the lights and highlights occupy a majority of the curve (roughly level 47 to 255), so while this curve seems extreme, levels 0-47 (shadows to the midtone) are mostly untouched.
To adjust the knee simply adjust the Curves Adjustment Layer. To reduce the highlight compression drag the right most point towards the top right. To increase the effect drag the right most point towards the bottom right. Note that to avoid the curve from arching in the center, and extra control point (the black dot in the image below) will need to be added and adjusted.
