Introduction / Migration
Transitioning Development from Release 5 to Release 6
Primary development with FFDD 5 was done with a collection of brightness adjustment layers followed by a collection of contrast adjustment layers, using Layer Masks to localize the effects. A common method was to increase the brightness in the darks, then use separate contrast adjustments for the darks and the lights, usually increasing the contrast more in areas of which the brightness was increased. Vincent Mounier has a great description of the process on his blog.
I had designed this multi-layer brightness and contrast process originally for black and white development to provide the ultimate in tonal control. When developing color however, this process can be tough to master because of how the brightness adjustment modifies the ratio of the primary colors (Red, Green, and Blue), which can result in shifts in color and saturation. Proper use of graduated contrast adjustments became necessary to correct for the color shifts.
Release 6 Provides an Easier Method
This sixth release replaces the brightness actions with two new exposure adjustment tools, dodge and gnd. Exposure adjustments, unlike brightness adjustments, do not modify the ratio of the primary color channels and thus will never cause color shifts. These new exposure adjustments in most cases work well with only a single uniform contrast adjustment for the entire image.
Here is an example. The top row has no contrast adjustments, and the bottom row has one single common increase in contrast. The first column is from the raw file unadjusted, the second (middle) from a single uniform FFDD6 exposure adjustment (done in a linear colorspace), and the third (right most) from a similar uniform FFDD5 brightness adjustment (done in the non-linear ProPhotoRGB colorspace). See how the FFDD6 example maintains the color. Doing the same development in FFDD5 would have required multiple contrast adjustments and a saturation adjustment.
Exposure adjustments, unlike brightness adjustments, can cause highlight clipping however. To combat this problem FFDD6 provides highlight control through the knee action.