A split neutral density filter...on steroids!
Outclass your glass with masking and fancy mousework

By Diane Miller www.DianeDMiller.com

I shot this picture on a frosty morning at sunrise in Teton National Park. It is a classic case of the shaded foreground needing more exposure than the mountains, illuminated by the first rays of dawn. But Photoshop can work wonders for it. The original (shown here) may look unsalvageable, but look at the finished result right below it, and see how I saved it in Photoshop. You can, too. Download a larger version of the original, open up your image editor, and follow along with me as I show you how. BEFORE AFTER

I could have used a split neutral density filter to darken the sky when I took the picture, which would have allowed more exposure to make the foreground lighter. But this still would have left the barn lost in the same tonalities as the base of the mountains, and I wanted to bring it out. I knew I could accomplish this with great control in my digital darkroom.

Making a digital split neutral density filter is an extension of the technique I used in a previous tutorial, "Digital Fill Flash." I showed you how to select a specific area of an image to modify, using Quick Mask mode, then how to use this area as a mask on an adjustment layer. The power of this method is the precision and flexibility you have in choosing the exact area, no matter how complex its shape, and being able to tweak it any time you want to. The technique is a great way to do a digital split neutral density filter, because you are not limited to the graduated edge being a straight line. The figure below on the left shows a typical split neutral density filter, and the one I'll show you how to create is on the right.



I'm using Photoshop CS2. You may be able to use other image editors, but you'll need one that allows masked adjustment layers. (Some programs have workarounds for some features they don't support directly.) Of course, menus will be different in different programs and versions, and I'm speaking Windows here--if you use a Mac, substitute Cmd for Ctrl and Option for Alt.

First, darken the sky

The first thing I did was to darken the sky and tops of the mountains with a simple split neutral density filter effect. I made a masked adjustment layer for Levels, drawing the mask with the Gradient tool to emulate the filter. To do this, I hit the Q key to go into Quick Mask mode and chose the Gradient tool, making sure I had the linear gradient option. I set the foreground color to black by clicking the Default Foreground and Background Colors icon at the bottom of the Tools palette--the small black and white squares. (Shortcut: Hit the D key.)

Then I used the Gradient tool to draw the mask by starting it just above the peaks, pulling the cursor straight down, and ending just below the top of the barn. As long as you move the cursor close to horizontal or vertical you can constrain it by holding down the Shift key. Here are the starting and ending points of the cursor. Above the start point the mask is 100 percent opaque, and it tapers to zero percent at the end point.



When I released the mouse button (or, since I use a Wacom® tablet, when I picked up the pen from the tablet) I got the mask shown below. It is 50 percent opaque so you can see underlying detail, but the 50 percent represents a 100 percent selected area. It's hard to see the feathered edge due to the color of the mountains, but it is shown on the left of the black and white figure above.



Then I hit Q again to get out of Quick Mask mode and I had the selection that matched the mask. If I had left my Quick Mask settings at their default, the area where I painted the mask would have represented protected areas, and I would have had to inverse the selection (Select > Inverse.) But it is simpler in most cases to draw a mask on the area I want selected, not protected, so I set my preferences for the mask to represent selected areas. You can do this by double-clicking the Quick Mask icon at the bottom of the Tool palette. The dialog box that comes up is self-explanatory.

I clicked on the half-black/half-white circle icon at the bottom of the Layers palette to create a new fill or adjustment layer, and made a Levels adjustment layer and moved the left slider over to the start of the histogram. (If this darkens the darks too much, use a combination of moving the left and middle sliders.)

This darkened the sky and mountain peaks nicely as you can see below.



Darkening a sky has pitfalls, though. Since there are only a few shades of tonality in the sky, I had to be careful not to darken it too much or I would posterize it. That is a subject for another tutorial, next month.

Lightening the foreground

Then I tackled the barn and foreground, which I wanted to lighten. I could do this most simply by using the inverse of the sky selection. I loaded the Levels 1 mask as a selection by clicking on its layer to make it active, and going to the menu bar and clicking Select > Load Selection > OK. Then I inversed it with Select > Inverse. Then I made a Curves adjustment layer as I did for Levels above, and lightened the foreground by dragging the middle point of the curve up. For an "average" image this would be a drastic move, but in this case it is ok because there aren't any light tones in this area to blow out (over-lighten). I then tweaked a point near the bottom of the curve back down a little, to keep the dark tones from going too light.

I used Curves here to allow me this added refinement of holding down the dark tones. Trying to do it with Levels would be more crude--the only way to darken the darks would be to bring the left slider in past the start of the histogram, blocking up the delicate, very dark tonalities to pure black. It is very seldom you want to do something like this, and it is one of the most common mistakes I see with image adjustments. It creates a gritty, harsh transition from medium-dark tones to pure black areas.

Now I have what the image would look like if I had used a real split neutral density filter. The brighter exposure on the bottom would have occurred in camera, with the filter holding back the exposure on the sky and peaks.



Now, add steroids

This is certainly an improvement over my starting image, but I wanted to brighten the top of the barn more. The mask is partially opaque there, holding back the full brightening effect, so I needed to carefully erase the mask over the barn.

I made sure the Curves layer was active (blue), then made the mask visible by going to the Channels palette and clicking the visibility icon (eyeball) of the bottom layer, which represents the mask. (You will only see this mask representation when a layer containing a mask is the active layer.) You don't have to make the mask visible to paint on it, as you will see below, but in this case I did so in order to erase it very precisely to match the shape of the barn roof.

When you make the mask visible it will show the protected areas in red, regardless of whether you have the default for drawing the mask set to representing protected or selected areas. With the mask visible I made my brush white by clicking on the Foreground / Background Color icon near the bottom of the Tools palette. (A black brush paints on the mask and white erases.) Then I zoomed in and painted away. I chose a brush size and hardness that let me begin with large brush strokes; as I got close to the roofline edges I went to a much smaller brush. But I didn't use a hard-edged brush--that would make an artificial-looking edge. The edge of the roof has a subtle softness I need to match.

In the figure below you can see I'm removing part of the mask. The circle is the brush, which is set to zero percent hardness. You can also see the Channels palette and the mask channel.



I can go back to the Layers palette and Alt-click on the mask icon to see just the mask, in black and white, to let me clean up details I can't see well in the translucent red mask view.

I can't get an absolutely perfect match along the roofline, where you see the brush in the figure above, but I don't need to. It would look bad if I erased the mask outside the roof. That would leave a light fringe around the barn. But if I stop just short of the edge it won't be obvious unless I make an adjustment that is far too strong. It isn't difficult to get it right because I can go back to it and keep tweaking it as often as I need.

When I was done I turned off the mask visibility in the Channels palette and went back to the Layers palette, where I clicked the visibility of the adjustment layer on and off to see if I had done a good job on the edges.

Fully revealing the barn top didn't make it light enough, so I added another masked adjustment layer to further lighten this area. I went into Quick Mask mode again, chose a black brush, and painted over the barn in red, outlining the edges carefully once again. I started with the edges, then went to a larger, harder brush to fill in. Then I looked at the mask in black and white, as above, to see if I needed to clean up any flaws.

I drew the mask all the way to the bottom of the barn, but I wanted a soft transition, fading to no brightening effect at its base, so I needed to choose a very large soft white brush to erase the mask in this area with a gradient effect. But I wasn't sure how much to erase until I had adjusted the brightness, so first I came out of Quick Mask mode to create a selection, then made a Curves layer, which automatically had the selection incorporated as a mask. I brightened the area by pulling the center of the curve up, and held down the dark tones just a bit, using a curve very similar to the first one, above.



Some final detailing

Now I was ready to exclude the bottom of the barn from the brightening effect. Here's a neat trick: In this case I could best judge where to paint by looking directly at the changes to the image while painting, not at the mask itself. But I was still painting on the mask. I made sure the Curves 2 layer was active, chose a large, black, soft brush, and just painted away, working from the bottom of the barn up, and watched the effect of the brightening adjustment be wiped out. In the figure below you can see what a large brush I used. My first attempt, with a 100 percent opaque brush, was too strong, even with such a large, soft brush, which is mostly transparent at the edges.

(Don't confuse brush opacity with hardness. A hard brush has no feathering on the edges, but can vary from 100 percent opaque to fully transparent. As the brush is made softer, using the slider in the Brush palette, it becomes more transparent at its edges, fading to zero there. But its center can still vary in opacity from 100 percent to zero, depending on the opacity setting.)

I hit Ctrl-Z to undo the brush stroke and reduced the brush opacity to 50 percent. Then I just made one swipe, just catching the bottom edge of the barn with the edge of the brush, and got the effect I wanted.



Now I felt the mountain peaks were a little light, so I went back to the Levels layer by double-clicking its adjustment icon (the one just left of the mask thumbnail) and pulled the middle slider a little more to the right. That is the beauty of adjustment layers--they are infinitely adjustable filters through which the image is seen.

When adjusting an image this strongly (either lightening or darkening) it is best to work in 16-bit mode. I didn't go into this in order to keep things simple. I'll touch on that more in the next tutorial dealing with the pitfalls of darkening a light area such as a pale sky.



Diane Miller is a widely-exhibited freelance photographer who lives north of San Francisco, in the Wine Country, and specializes in fine-art nature photography. Her work, which can be found on her web site, www.DianeDMiller.com, has been published and exhibited throughout the Pacific Northwest. © 2006 Adorama