A Split Neutral Density Filter on Steroids
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 was so
pathetic I’m going to show you the “after” version, before you decide you have better
things to do.
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 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.
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Making a digital split neutral density filter is an extension of the technique I used in the
tutorial “Digital Fill Flash.” That showed 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 hit the Q key to go into Quick Mask mode and chose the Gradient tool, making sure I
had the linear gradient option. I made sure the foreground color was black by clicking the
Default Foreground and Background Colors icon at the bottom of the Tools palette – the
small black and white squares – or you can just hit the D key. Then I drew 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.
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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 a 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 and selecting the option for Color Indicates Selected Areas.
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 in to the start of the histogram. (On images that have no blacks this will darken
the darks too much. If that happens, use a combination of moving the left and middle
sliders.) You can also use Curves for this adjustment.
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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. As much as
possible this kind of adjustment is best done in Adobe Camera Raw or Lightroom, where
you will get much less posterization because you are working on the full set of tonalities
the camera captured.
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 (overlighten.) I then tweaked a point near the bottom of the curve back down a little, to keep
the dark tones from going too light.
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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.
[Fig 10]
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 hitting the
Backslash key. 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. If the mask color is not easy to see, double-click the Quick Mask icon and
click the color square to change it.
When you make the mask visible it will show the protected areas colored, 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, but 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.
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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. (The Channels palette is another way of seeing the mask.)
I can go back to the Layers palette and Alt-click 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 colored 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 finished I turned off the mask visibility with the backslash key (or 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
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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. Here is the result.
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.
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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. Changing the settings
does no damage to the image, unlike making tonal adjustments directly on the pixel layer.
When adjusting an image this strongly (either lightening or darkening) it is important to
bring the image from the raw converter into Photoshop in 16-bit mode.