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Digital Blending with a Layer Mask

The variations of subject brightness encountered when doing landscape photography often exceed the range that can be captured with an acceptable exposure. Your eyes can see between fifteen and thirty stops of brightness in a single scene, but the camera is limited to not more than about five. The traditional way of dealing with this problem is the use of graduated neutral density filters to darken down the sky enough to fit better with the land. But the digital darkroom gives us a new solution: merging two exposures using digital blending.

To make use of this technique, you have to plan for it when you are in the field with your camera. When shooting, don't even try to get the entire scene exposed correctly in one shot. Instead, take two shots, one with the foreground exposed correctly, and one exposed for the sky. Viewed one at a time, both shots will look pretty bad. The one exposed for the foreground will likely have burned out skies, and the one metered for the sky will have blocked up shadows covering the land. Once both versions are opened in Photoshop though, you can take the good parts of each and merge them together to create an image that looks the way your eyes actually saw it.

Darker exposure shot for later digital blending
Darker exposure shot for later digital blending. The sky looks good here, but the foreground is lost in shadows.
Lighter exposure shot for later digital blending
Lighter exposure shot for later digital blending. The landscape itself looks much better in this version, but the sky is burned out.

To actually merge your exposures, first copy and paste the darker of the two shots on top of the lighter one. As with most things in Photoshop, there are several ways to do this. Using the menus, you can use Select >> All followed by Edit >> Copy on the darker shot, followed by Edit >> Paste on the lighter. Or if you prefer keyboard shortcuts, you can go for Control-A to select the entire image, followed by Control-C to copy it to the clipboard, and then use Control-V to paste it on top of the other shot. Or, those who prefer the mouse can do the same thing by dragging and dropping the darker image over to the lighter one while holding down the Shift key to ensure that ends up correctly centered. Take your pick, as all three methods accomplish the same thing.

As you no doubt have noticed, the darker image has now completely hidden the lighter version underneath. To reveal selected portions of the lighter shot, we'll use a layer mask on the top layer. As we've done with several other tips lately, you can add one by selecting the darker layer and clicking on the "circle within a square" new layer mask icon at the bottom of the Layers Palette or by using the equivalent menu commands. Then, with a black brush, paint on the mask to reveal the lighter version underneath.

Getting things to look natural can take a fair bit of time and some degree of experience. But since the actual pixel data of both layers is never altered, you can paint and repaint the mask as much as needed until you like the results. Some images will be easier to merge than others.

The result from manually blending the two exposures
The result from blending the two exposures using a hand-painted layer mask
The layer mask used
The layer mask used

A bit of thinking about what we're trying to do here can led to the discovery of a great trick for creating the basis of the needed mask. What we want is for the lighter image to show through anywhere where the top image is too dark. That is to say, we need to hide the darker portions of the darker image. Since black hides in a layer mask and light reveals, we can actually use a black and white version of the image itself as a layer mask. Look at the lighter background version for a minute. The sky is likely mostly burned out, which when used as a mask on the top darker layer will allow the better version of the sky to show. We exposed the bottom image for the land, so a grayscale version of the foreground used as a mask will hide much of the darker top image's land, just as we want.

If you're with me so far, let's look at how to actually do this.

Click on the bottom layer to make it active, then press Control-A to select the entire layer and press Control-C to copy it to the clipboard. Now click on the top layer so that it is active, then click on the new layer mask icon at the bottom of the Layers palette. Then hold down the Alt key and click on the empty layer mask you just created to view it as the active image. Now press Control-V to paste the copied bottom layer into the mask. Photoshop will automatically convert it to grayscale since a mask can't contain color.

Now that you've created the mask, you need to blur it just a bit to smooth out the transitions between light and dark to take the harsh edges off. Click on the main image icon for the top layer to view it instead of the mask. Now click back on the mask to make it active again while keeping the image viewable. Then select Filter >> Blur >> Gaussian Blur and adjust the Radius just enough to smooth out the mask a little. Too much blur will result in ghosting around edges in the image; too little will leave you with unnaturally sharp edges. If you are doing this correctly, the Preview image in the Gaussian Blur dialog will be in grayscale. If yours is in color, you have accidentally selected the image layer rather than the mask, so cancel out and click on the mask, then try the blur again.

Chances are that using the grayscale version of the image itself will get you in the ballpark for a mask faster than you can do with paintbrush, but don't feel like you have to stop there. A bit of manual touch up can make a good mask even better. Zoom in and examine the merged image closely to be sure you are happy with it. You may also sometimes find it helpful to adjust your mask using Levels or Curves to alter its brightness or contrast.

The result from blending using a grascale copy as the mask
The result from blending using a grascale copy as the mask.
The layer mask used
The layer mask used. Notice how it looks similar to the mask I painted by hand.

Personally, I still prefer getting as close as I can with graduated neutral density filters in the field. If I can capture a scene in a single shot, I won't have to worry about anything moving between shots that would make a digtal merge more difficult. Out in nature, the wind does blow so movement is always a worry when shooting multiple exposures. Digital blending though does free you from problems inherent in trying to line up the straight edge of a filter graduation with an uneven horizon line. It's worth being proficient with both ways of dealing with extremes of subject brightness so you can use whichever best fits a given situation.


Date posted: April 29, 2007

 

Copyright © 2007 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: Creative Sharpening with a Layer Mask Return to archives menu Next tip: A Pocket Full of Compact Flash Cards

Related articles:
Essential Filters: Neutral Density and Graduated ND
Cokin P-sized versus the Larger Lee-sized Graduated ND Filters
Layer Masks on an Image Layer
Are Graduated Neutral Density Filters Obsolete in the Age of HDR?
 

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