Development / GND Filter
Actions
- gnd/setup : Creates a new custom graduated neutral density filter (exposure adjustment).
The gnd tool provides a powerful and easy non-destructive method of applying one or more graduated neutral density (GND) filters to an image. This tool provides much more flexibility than a physical GND filter applied in the field, in that it gives complete control of the shape and amount of graduation of the filter. One other advantage of this tool is that it functions using pure adjustment layers without any layer masks, so it does not use extra memory when editing and disk space when saving.
A real GND filter produces a graduated darkening of a scene, effectively reducing the overall dynamic range. For example, one might wish to expose for relatively dark land, while simultaneously using a GND filter to darken a bright sky, thus allowing for proper exposure of the whole scene. If no GND filter was used, parts of the sky would most likely clip to white. Note that a GND filter does not effect the contrast of the photo, but instead it effects exposure only. Also another key concept to understand is that this exposure adjustment is different than a brightness adjustment. You can cause your highlights to clip with a GND filter or an exposure adjustment, while a brightness adjustment changes the tonality without causing any highlight clipping. More on this later...
The typical FFDD blended extended dynamic range negative looks under exposed. This is due to the very large difference in luminosity between highlights and midtones, and the much smaller difference in luminosity between the midtones and shadows. Given that the blended negative is dark, applying a graduated darkening of the scene does not make sense. Instead the FFDD GND tool works backwards and lightens the image.
On to the Example
The un-developed blended digital negative below, blended with the batch script, is going to be used in this example. The smaller develop -D file was loaded to enable quick development which can be applied to the full size digital negative later.
This photo was to become a 5:2 aspect ratio pano, so a Color Fill Adjustment layer was added to mask off the top and a little of the bottom to make development easier. Note that with the batch script the image must stay un-cropped in order to generate the full size image later.
Below is the histogram of the digital negative in its undeveloped state. Keep in mind that this image is being edited in a linear colorspace so the midlevel 50% gray is level 47 instead of level 128 (which would be the logical center). So the histogram will always look left weighted.
Next a little contrast was added using the linear colorspace contrast actions. The under-exposure of the image becomes more pronounced as the contrast expands the mid-tones and compresses the highlights and shadows. To save space, the image below is only showing the non-masked area of the photo.
To correct the exposure of the image the gnd action will be used. Make sure to apply the GND before the contrast adjustment layers.
Use the gnd/setup action to create the following arrangement of adjustment layers. The bottom layer controls the direction of graduation and will be covered later. The top layer controls the shape of the graduation and amount of exposure adjustment. Also note that top layer must remain as a Clipping Mask (note the arrow) in order for it to function properly.
The gnd/setup action when finished will bring up a Curves adjustment which controls the shape and amount of the graduation. For now just accept the default and click the OK button.
Here is the result of the default GND settings on the image.
To see the graduation, try turning off the background layer. The image will disappear and instead the graduation can be seen. The whiter the graduation, the more the GND filter will lighten the image via its exposure adjustment.
Controlling the Graduation Shape and Amount
Double clicking on the gnd/curve adjustment layer will allow for adjustment of the graduation, and will bring up a Curves Adjustment Layer control.
Normally a Curves Adjustment Layer is used to control the tonality of an image. In the case of the gnd action it is being used instead to control exposure in the direction of the graduation. The best way to understand how it works is to just try it on an image and then play with the curve shape.
Below is an example of just playing around with the curve. First off, the up/down axis controls the exposure. The closer the curve is to the top, the brighter the image will become. The left/right axis shows the change of the exposure in the direction of the gradient. The default direction of the gradient in the gnd action is top to bottom. So the top is the left most part of the curve and the bottom is the right most part of the curve. For best control make sure to use the larger curves control which can be toggled by 6 the corner button.
Point 1. This is the bottom most section of the gradient which is brightened as much as possible. Note that since exposure adjustments can clip, this area has gone to full white.
Point 2. The point where the gradient starts.
Point 3. Using the mouse to click on the image will display a locater circle on the curve showing which point on the curve controls which part of the gradient. This is a good way to quickly find areas which need adjustment.
Point 4. The point where the gradient ends and no exposure adjustment is being made. Note that it was set to line up with the point at which the white color fill starts to show the 5:2 crop.
Point 5. This is adjusting areas outside of where the final 5:2 crop will be.
Ultimately the curve below was used to develop this image. The upper part of the sky was brightened (the left hump in the curve), with the lower part of the sky a little darker (the middle dip in the curve). As for the bottom, the area closer to the horizon was kept bright highlighting the long exposure blur of the tide (the right hump in the curve) and the lower half was kept dark to show the yellow reflection in the standing water.
Resulting in the following image. Note the histogram is now much more spread out, but will still have a left bias because of the linear colorspace used for editing.
Drawing the Graduation
One awesome feature of Photoshop's Curves Adjustment Layers is the pencil option, which allows the curve to be directly drawn instead of trying to mess with control points. Sometimes this can be easier to use. Below is an example of a very odd graduation built with the pencil tool. See the two peaks in the curve and the corresponding bright areas in the photo in the background.
Once finished, the Smooth button can be used to smooth out the curve.
Controlling the Direction of Graduation
Double click on the gnd/direction layer to control the direction of the graduation. Note the default graduation is up and down (90 degrees). This will bring up the controls for the Gradient Fill Adjustment Layer shown below.
Change the Angle to adjust the direction of the graduation. This works much like rotating a real GND filter on the lens. In this case the direction was moved from portrait (90 degrees) to landscape (0 degrees).
Using the default GND curve with a angle of 0 results in a left to right graduation which can be seen below.
Also note that when using diagonal angles, sometimes Scale setting must be adjusted to spread out the gradient to fill the entire frame.