The graduated split neutral density filter has been a must-have filter for nature photographers for years. The filter basically has a dark gray top half and a clear bottom half, with a gradual transition from gray to clear, and comes in a variety of strengths and gradations. Placed in front your camera’s lens it allows you to capture detail in the bright highlights in the top half of a scene (usually the sky) and still capture detail in your shadow areas (I’ll be explaining how to do this in Week 9 of my on-line course.) With the advent of digital photography, photographers are able to simulate this effect using a variety of techniques, from HDR processing to the shadow/highlight adjustment in Photoshop to liberal use of the recovery and fill light sliders in Adobe Camera Raw.
For me, the easiest and most natural looking way to digitally simulate the use of a split ND filter is to take two photos of the same scene, one exposed for the highlights and one exposed for the shadows, and then combine the images in Photoshop. This is one of the first Photoshop techniques I used on a regular basis when I made the transition to digital imaging, and I find it is still a quick and useful technique. This procedure uses a layer mask, which I’ll explain in some detail, but if you’re not familiar with layer masks, you might want to read my previous post “Basic Photoshop – Layer Masks”
Step one is to remember to take two or more photos when you are out shooting. Use a tripod, so your images will align perfectly and take one exposure that captures all the highlight details and one exposure that captures all your shadow details. These can be 2, 3, or sometimes 4 stops apart. For the highlight shot, check your histogram and confirm you are not clipping any highlights. For the shadow shot, make sure you are not clipping any blacks. Above are two shots from a scene in western Connecticut.
To get started, open both images in Photoshop, then copy the bright image onto the dark image. The best way to do this is to use the move tool (top tool on the tool bar). Click on the bright image, hold down the shift key (this will align the images) and drag the bright image on top of the dark image. You can now close the bright image because we’ll be working with just the dark image which now has two layers, one bright, one dark.
With the bright layer highlighted in the layers palette, click on the layer mask icon at the bottom of the palette. Now any time you add black to this layer (using a paint brush, the fill tool, etc.) the mask will be covered, revealing the darker layer underneath. I used to spend time making a perfect selection of the sky area and then filling it with black, but it this is tedious and often looked kind of weird around the edge of the mask. Instead, I now use the gradient tool to create a graduated mask that better simulates the split ND filter.
Here’s how to use the gradient tool on your layer mask. First, make sure your foreground color is set to black (the easiest way to this is to just press the “D” key.) Second, select the gradient tool and make sure the black to transparent gradient is selected (you can find this on the gradient tool bar which will be under the menu at the top of the screen.) To draw a gradient, just click on the screen near your horizon line and drag down. When you let go of the mouse button, all the pixels above where you started the gradient will be filled with black. Everything below where you let go of the button will not be filled at all. Everything in between will be a gradual transition from black to transparent. (Tip: as you drag, hold down the shift key – this will keep your gradient line straight up and down.) Of course, since your drawing the gradient on the layer mask, it’s not actually adding black to your image, just the mask, revealing the darker image on the layer below the bright layer.
You’ll notice in this image that once I applied my gradient, the tops of the hills on the left and right were affected by the mask, darkening them in a way that looks unnatural. It’s easy to fine-tune these types of problems using the brush tool. By painting over these areas with white, you can basically erase the black the gradient added to the mask.
By changing the foreground color to white and using the brush tool, you can easily erase the changes made by the gradient tool in only those areas you want to fix. A couple of things about using the brush tool for this technique. First make sure your brush has hardness setting of 0 – this will give you a nice feathered brush and will give you a more natural look on the edges of where you are painting. Second, as you fine tune your mask, painting with white or black to reveal or cover the mask, you can get as nuanced as you want by changing the opacity and/or flow of the brush in the brush tool bar. This lets you paint in shades of gray, instead of black or white, giving you the ability to create really natural looking transitions.
Please post any questions about this technique in the comments section below, and feel free to suggest future post topics.