Thursday, March 1, 2012

Adobe Photoshop Lightroom Tutorial - Part XIX - Graduated Filters

Part I - Index and Introduction
Part XX - Exposure vs. Brightness

The second Lightroom tool I'd like to talk about is the graduated filter.

Physical graduated filters are devices commonly used by photographers, usually to darken the filtered parts of an image whose luminance level wouldn't otherwise fall into the dynamic range of the film or the sensor.

Adobe Lightroom brings the same concept into the post-production domain and further enhances it. Lightroom's graduated filters are fully configurable and let you apply different kinds of adjustments:
  • Color temperature and tint.
  • Basic tone controls: Exposure, contrast, highlights and shadows.
  • Clarity.
  • Saturation.
  • Sharpness.
  • Noise.
  • MoirĂ©.
  • Color.

Furthermore, they allow you to apply both positive and negative adjustments, contrary to what can usually be done with physical graduated filters which can only darken the filtered image.

To add a graduated filter to an image, or modify an existing one, you can use one of the following techniques:
  • Select the Tools/Graduated Filter menu item.
  • Select the graduated filter icon on the tool strip in the Develop module (described in the previous post).
  • Use the 'M' keyboard shortcut.

The graduated filter panel will open as shown in the following screenshot.

Graduated Filter Panel

In the upper part of the panel (called Mask) you can choose whether you're adding a new filter (New, the default choice when you open the panel) or if you want to modify an existing one (Edit). Graduated filters are fully configurable using the adjustment sliders you can find on the main panel window.

You don't need to always start a filter configuration from scratch, though. You can choose an existing filter from the Effect drop-down menu (Lightroom bundles some of them) and even create your own saving the current configuration of a filter into a new preset, using the Save the current settings as a new preset menu item. That's a pretty handy feature if you use similar settings over and over again.
Also, since you will often need to fine tune the filter settings after applying it, you can safely add a configured filter, check the results and fine tune its adjustments according to your needs.

How a Graduated Filter Works

A graduated filter is defined not only by its effect settings, but also by the following parameters:
  • Its position on the image and its inclination.
  • Its direction, or its orientation.
  • Its width.
The meaning of each of these terms is summarized in the following picture and is explained in detail in the following sections:

Characteristics of a Graduated Filter

The position of a filter just indicates where it's located on the image. Its "centre", indicated by the grey handle, is just a convenience for you to spot the existing filters on an image: as far as you move it along the filter axis (the middle line), the results won't change.

As most physical filters, a Lightroom's graduated filter applies effects whose intensity depends solely on the distance of a point from the filter axis (on other words, the intensity is the same along a line that is parallel to the filter axis). The inclination of the filter is the angle between its axis and the horizon and it's a critical parameter that determines how the chosen effect is applied to the image.

The width, determined by the distance between the filter axis and either of the other two lines that define the filter, is the width of the graduated zone of the filter. The larger the graduated zone, the softer the edges of the filter and the smoother degradation of the filter effect from one of its sides to the other.

In fact, a graduated filters applies its effect in three different ways on three different zones of the image that are determined by the its axis and its graduated zone of the filter. When you lie a filter on an image, its axis divides the image into two sides, that we will label inner and outer sides: the inner side is the side of the image that contains the point you started dragging the filter from, the other one being the outer. The filter behaviour outside the graduated zone is the following:
  • In the inner side the effect is applied with full intensity. In the previous image, for example, this zone is identified by the blue arrow.
  • In the outer side, the filter has no effect.
Inside the graduate zone, the intensity of the filter linearly falls off from 100% to 0% moving along the line perpendicular to the filter axis from the inner side to the outer side. This line, and the direction of the falloff, is indicated on the previous image with a yellow arrow.

Modifying a Graduated Filter

Any parameter of an existing graduated filter can be modified. To select a filter to modify, you've got to perform the following operations:
  • Open the graduated filters panel as explained above.
  • Select the filter you want to modify by clicking on its handle (the grey circle in its center).
  • Apply any modification you want to the effect sliders.

Similarly, you can modify the geometric characteristics of a filter by clicking and dragging:

  • Its handle, if you want to move it.
  • Its axis, if you want to rotate it.
  • Its graduated zone boundaries, if you want to change its width.

Common Uses

Historically, physical graduated filters have been used to reduce the light intensity in selected parts of an image so that all of it falls into the dynamic range of the film or the sensor being used and avoid burning it out. A common case is using a filter to reduce the intensity of the light coming from the sky. Since data from a  clipped channel cannot be recovered in post production, this type of filters is still widely used on camera. Nevertheless, it's often convenient to apply an exposure correction with a graduated filter in post production, provided your channel is not burnt out.

In the following image, I used two graduated filters: the first one was used to slightly darken the exposure of the sky and give it a soft gradient, the other one to recover detail in the shadows that were projected over the square by the building behind me:

Final Image

The first graduated filter is configured to increase the exposure by 1 f/stop and, as you can see in the following screenshot, its axis lays just along the border of the shadow and its graduated zone is very narrow.

Graduated Filter - Exposure: +1

The second graduated filter is configured to decrease the exposure of the sky only slightly, by approximately 1/3 f/stop and, as you can see in the following screenshot, its axis lays just along the border of the building in foreground and its graduated zone is very wide, so that the gradient in the sky is very smooth and soft.

Graduated Filter - Exposure: -0.3

The original picture without graduated filters is the following:

Original Image

As you can, the underexposure of the shadows couldn't have been corrected using the tones controls: since their effect is global, shadows on the other parts of the image, such as the foreground building, would have been compromised.

Please notice that there are two color temperatures on the image: the temperature of the sunny part and the temperature of the shady part. On one hand, the graduated filter I used to raise the exposure of the shadows has emphasized the details on that part of the image. On the other hand, however, it has also accentuated the difference between the two different temperatures and now you can clearly see a blue dominant in it. This problem, before Lightroom 4 was available, could only be corrected using an external image editing program. Instead, Lightroom 4 allow us to apply non global color temperature adjustments by means of both graduated filters and adjustment brushes.

Conclusion

Graduated filters are a very convenient way to apply an effect to a section of an image whose boundary is  a straight line, such as horizons, straight buildings, rectilinear shadows, and so on.

In this tutorial, we've explored how a graduated filter works and how it can be used to fix the exposure in part of an image. However, as stated in the previous sections, Lightroom will let you apply many kinds of adjustments through a graduated filter giving you a great flexibility when it comes to post processing your images.

In the cases in which a graduated filter isn't suitable, an adjustment brush can be used instead, as I will explain in the upcoming part of this tutorial.

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