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Getting it right on film (or on the sensor): Graduated Neutral Density Filters

A graduated neutral density (ND) filter, sometimes called “grads” is the traditional tool that film shooters utilize to “even out” the exposure in a scene. These filters are usually plastic (resin), and come in a variety of forms, but the good ones are rectangular and mount in a special holder that screws into the front of your lens. The filters themselves are color-neutral (grey), and the tint is a gradient from dark to clear. Each filter is rated to a certain number of f-stops, and have either a hard (abrupt) or soft transition from dark to clear. For example, a three-stop soft ND grad will reduce the light intesity by three stops at the top, and then gradually taper to clear. ND grads are great in that you can put the dark part of the filter on the bright part of your scene, thus darkening it, while leaving the shady part of the scene alone. However, there are some functional limitations with ND grad filters. It is often quite challenging to properly align ND grads in the field, they can scractch easily, and many prevent the use of lens shades (hoods). In addition, the design of ND grads makes them great for scenes with a flat horizon, but what if you have large foreground objects or mountains in your scene that you don’t want to darken? Or worse, what about the classic "mountains reflected in a lake" picture-- you can end up with the reflected image being lighter than the actual mountains-- impossible! Fortunately for us. post-processing software comes to the rescue!

Using Photoshop to create virtual ND Grad filters

Photoshop comes with the standard Dodge and Burn tools which work exactly how you might expect them to in a real darkroom. However, dodging and burning can get kind of clunky and the results can easily look artificial if you use the wrong kind of brush. Fortunately, there’s a pretty easy way to re-create the ND grad filter in photoshop! First, make sure you followed the rule of exposing for the highlights in your original image. Blown highlights will not be recoverable, no matter what kind of Photoshop guru you may be.

Bring your image into Photoshop, and make sure the Layers palette is open. Create a copy of the background by clicking in “Duplicate Layer” from the Layers palette menu. You now have two layers of the same image. Select the new layer in the layers palette and bring up your Levels control. Adjust the levels so that the shadows and midtones are pleasing. Don’t worry about the highlights—let them blow out. Good, you're halfway there. Now create a layer mask by clicking the Layer Mask icon in the Layers palette. Make sure the mask is selected in the Layers palette (a little box will appear around its icon) and set your foreground color to black and your background color to white. Now choose the gradient tool (you can click the “g” key on your keyboard) and think about where you would like to recover your highlights. Draw a gradient in your layer mask from top to bottom. You’ll want to experiment with the placement, but remember that any area that is “black” will disappear from your layer, allowing the background to show through. Congratulations, you’ve just created a virtual ND grad filter.

What if you have mountains or foreground objects that you need to lighten? Easy enough—use the paintbrush tool and set the flow to 20% or less. Choose a large, soft-edged brush and paint in the layer mask. Choose white to lighten, or black to darken.

Here are some before and after shots taken to illustrate the power of this technique:

Open space in my neighborhood, with Pikes Peak in the distance. Since it's late afternoon and I'm facing west, we get a challenging, backlit exposure situation. I made sure to expose for the highlights to retain detail in the clouds, but our foreground is a little too dark.

Here's the same shot with our virtual ND grad applied. Now the foreground has some pop, but the clouds still have their detail.

This image works well with our virtual ND grad because we are able to easily lighten up the houses in the background with the brush tool in the layer mask. However, there are lots of situations in nature, such as trees in the foreground, where trying to use an ND grad and the dodge/burn brushes become an exercise in futility.

Next Page: HDR Merge in Photoshop CS2 for those REALLY challenging situtions.

Lupine field, Crested Butte, CO. I used a ND grad filter to balance the exposure of the sky, but it also darkened the mountains. I had to dodge a little in Photoshop to get the desired effect.

Create a duplicate layer from the background in the Layers palette.

Layers palette in Photoshop CS2 showing the layer mask with gradient and dodging applied. Note the black corners in the mask icon, indicating that it is selected.

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