The use of graduated neutral density filters (also known as split ND filters) is essential for landscape photography. Films and digital camera sensors are not able to record the wide dynamic range that our eyes enjoy in the outdoors. Before going digital, I used to work with slide film — which had a latitude range of about 5-stops of light. In high contrast situations, such limited range can be an image killer. The photo at right illustrates this problem. This particular photo was exposed for the foreground reflection. Due to the high contrast, the mountain was washed out — with details completely lost in the highlights. For advanced amateurs and pros who display and sell their work, this is a serious problem. Fortunately, there is a digital darkroom technique (which combines easy "in-the-field" and digital darkroom steps) that overcomes this common exposure problem, and can add two or three stops to the range of light in your image.

Can a well-balanced exposure be achieved through other methods, including a real ND filter? Of course. Some will prefer carrying real filters over using this digital darkroom technique. Some will find other ways to edit the image. For example, the image could have been exposed for the mountain, leaving the reflection to be recorded as a fairly dark mass. The resulting image then could have been adjusted in image editing software, by isolating and "lightening" (dodging and burning) the shadow areas. However, this technique usually adds messy, undesirable image "noise." The noise problem can show up as multi-colored artifacts in digital images, or as emphasized grain in the low contrast, low color shadows areas in the scanned image. The technique described on this page avoids noise problems completely.

I rely upon a digital darkroom technique, using basic layering and masking, that allows me duplicate the effect of the graduated ND filter — without fumbling around with filters in the field. For people who want to simplify their shooting process (or who want easier shoots in variable weather conditions such as the cold of winter), this technique is worth learning. This process has worked for me for many years. In fact, I do not carry these filters in the field anymore. I simply shoot two different exposures in the field, one metered for the highlights and one metered for the shadows. I then "blend" these two exposures with software at home. The result is an image that has the same outcome as if I had used an ND filter in the field. In fact, the digital technique produces more desirable and natural results, as I am not limited to a single hard or soft graduated edge in my composition. We've all seen photos where an obvious "edge" marks the use of a graduated ND filter. In my opinion, this takes away from the beauty recorded in the photo. I am a firm believer in making sure that photos do not reveal the use of filters whenever possible. This digital ND technique allows me to do just that.

This is a technique that has been described by many photographers and teachers. I am not trying to reinvent the wheel. I have simply found a particular process that is simple, fast and reliable for my landscape photography needs. There are more advanced methods that can be used (such as those that employ advance tone masks), if you find that this method does not achieve the results you want. I find that this technique works for 99% of my images. Furthermore, this technique requires software that uses layers (such as Photoshop), and a basic knowledge of working with layers (or a little practice). The steps outlined below illustrate the process I use with Photoshop.

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Digital Graduated ND Filter
Click any screen shot for a larger view.

Step 1 : open both exposures

Record two exposures of your scene in the field (one for highlights and one for shadows), making sure that your composition does not change. A tripod is necessary. Open the two digital files in Photoshop, or a similarly capable application. If you are scanning two slides, make sure that your scanning settings do not change between scans. Place the images side by side (figure 1) in Photoshop. Make sure your layers pallete is open.


figure 1

Step 2 : combine two into one

With the "darker" exposure window active, shift-click-drag the darker image into to the "lighter" photo window. This drag/drop technique automatically places the darker image on a new layer, centered on top of the lighter image. (You can also edit/select all and edit/copy the dark file, select the lighter image window, and then edit/paste the darker image into the lighter image file. It has the same result.) You can close the darker photo file after pasting. You now have one layered file containing both exposures open in Photoshop. Select the darker image layer in your layers palette. Within the layers palette, select and drag the Opacity slider to the left, to check the alignment of your layered exposures (figure 2). If the layers are aligned (using a tripod should have ensured this), then move on to the next step. Use the Move Tool to align the layers as needed.


figure 2

Step 3 : create a basic blend

This subjective step will take some practice — a step that duplicates the simple effect of using the ND filter in the field. As we all know, this is very subjective. To begin, first make sure your Opacity slider has been returned to 100%. Next, use the Selection Tool to select an area of the darker image layer that you wish to "lighten." (This step duplicates what you would be doing in the field, as you place the filter over your lens.) In order to create a smooth transition (much like a soft split ND filter), you will now "feather" the selection you just made. With the selection active, go to Select/Feather and enter a number (figure 3). This number represents the "length" of the softening effect. You will have to play around with this number, as the effect can vary — depending on file size and resolution. Hit OK. You will not see any changes in the selection at this point. Now, hit your delete key, or go Edit/Clear. The soft-edged selection you created on the darker layer will now be deleted, revealing the better shadow exposure in the layer below. (Advanced users can also use a layer mask after feathering the selection, instead of deleting the image data.)


figure 3

Step 4 : evaluate the blend

You have just created the digital equivalent of using a graduated neutral density filter in the field (figure 4). If your layered image features a noticeable blending edge (as the example photo does, in the rocky shore), proceed to the next step to fine tune and eliminate as much of the blended edge as possible.


figure 3

Step 5 : fine tune the blend

(This step requires basic knowledge of masking, or a little practice with the following technique.) I don't want the final image to reveal any filter edges, or hints of using an ND filter. Therefore, I want to "even out" the shadow areas affected by the blend (in this case, the rocky shore). Instead of dodging or burning (which might enhance nasty image noise), I will use a simple layer mask to even out the blend in the shadow areas.

Select the top layer in your layers palette. Go to Layer/Add Layer Mask/Reveal All. You will now "paint" on the new layer mask to conceal parts of the darker shadows of the top layer, near the blend edge. In your layers palette, click on the white mask icon next to the top layer icon. Any editing from this point on will affect only the mask, and not the underlying image. (This allows you to mess up without affecting the original image layer.) Make sure your foreground color is black. Select the Paintbrush Tool. Choose a soft edge brush (play around with sizes), and begin "painting" in the shadow areas that you would like to "lighten" near the blend edge. With this step, you are effectively "erasing" (realy just concealing) the darker shadows of the top layer. Again, this process affects only the layer mask. Undo, and start over with a new mask, if needed. Continue the freehand painting until you have evened out your shadow areas.

Alternative: You may also skip the layer mask step, and simply adjust (i.e. darken) the shadows of the bottom layer (using a Levels or Curves adjustment) to match the shadow tone of the top layer — if this achieves the desired results.


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Step 6 : final adjustments

If either of the layers need further editing (such as Levels/Curves adjustment), do so now. If you are satisfied with your digital ND blend, then flatten your image. Go to Layer/Flatten Image. (If you prefer to save your layers and/or mask for future editing, do not flatten the image. Simply save as a .PSD file.) After flattening, save your file. Then proceed with any final steps that you might normally use for image processing (including sharpening, Color adjustments, Levels/Curves adjustments, etc.). You now have a finished image with well-balanced exposure and dynamic range.


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Summary

After using graduated ND filters in the field for years, I am now relying solely on this digital darkroom technique to achieve the same results. My bag is a little lighter. I don't have any filters to lose, scratch or fumble with in winter. I can work faster in the field. I have more control over final image quality, which is not reduced by the use of straight-edged filters.

I find that this technique is generally usable for exposures that differ by two or three stops at most. If you try to push the range any further, an unnatural looking image may be the result.

If you have any questions, please use the contact link below.

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