Tucked away in the tool strip of the Develop module is the graduated filter tool. The graduated filter tool is another non-destructive local adjustment tool that made its debut with Lightroom 2.0 and contributes to Lightroom's power and flexibility.
The most obvious use of the graduated filter tool is to recreate the effect of an on lens graduated neutral density filter. The purpose of this type of filter is to allow the photographer to compensate for the tonal differences in different areas of a scene. For example, when shooting a landscape you may be faced with deciding to expose for the bright sky, thereby under-exposing the foreground detail. Or, you can expose for the foreground and wind up blowing out the sky detail. A graduated neutral density will block more light on one part of the filter and gradually let more light through as you progress across the filter. This allows you to expose the scene without blowing out the sky.
To access the graduated filter tool switch to the Develop module and click the icon in the tool strip on the right (just under the histogram) or press the letter M.
If you flip the small switch in the upper right corner you can access the sliders which will afford you much finer control over the tool. While you can work with the + and - buttons I find the sliders much more intuitive.
In order to make it obvious how the filter is applied, let's work on a pure white image and turn down the exposure and brightness. This will make it easier to see the applied transition.
To apply the filter click on the image and begin dragging. You soon learn that you can drag in any direction. To restrict yourself to 90 degree increments hold down the shift key while dragging. What you will see is three equally spaced lines with a pin in the middle of the center line. The pin looks like a small grey circle with a black dot in the middle.
To keep the middle line (the one with the pin) stationary and still expand the transition area, hold down the Option key on a Mac (the Alt key in Windows) and drag either the top or bottom lines. Â You can also use the Option key (Alt in Windows) to adjust all the applied sliders at once. Hover your cursor over the pin, press the Option/Alt key and the hand will turn into a vertical bar with arrows pointing left and right, Simply scrub (drag) left and right and all the applied sliders will adjust proportionally.
As the distance between the top and bottom lines the transition between full application of the filter to no filter becomes more gradual.
Of course, you can apply the filter at any angle you like in order to adjust your image. Â Once the filter is applied you can grab the middle line at any point except the pin. Your cursor will be a curved double arrow. Drag the line and adjust the angle at which the filter is applied.
When you have more than one filter applied you can only have one active at a time for adjustments. The unselected or inactive filters have an all grey pin and their corresponding three lines are not shown. The selective filter has the grey circle with a black dot pin along with the three lines.
While working with the filter the lines and pins can sometimes obstruct your view. The H key will toggle the visibility and let you see what you are doing.
So with a relatively simple adjustment you can bring back some detail in over-exposed areas while leaving others relatively untouched.