Blending two exposures in Photoshop
The Digital Grad ND Filter
This is a technique that essentially increases the dynamic range of your camera by taking two or more exposures, each optimized for specific elements of the image, and blending them in Photoshop or Photoshop Elements (or any image editing program capable of Layers). It's usually used for scenes that have a foreground in shade and a sunlit background, something that often happens at sunrise or sunset.
First of all, all exposures must be of the exact same composition. This means using a tripod. Though you can do this without a tripod, you'll spend more time in Photoshop lining up the images than it would be worth. So do yourself a favor and use a sturdy tripod.
Determine exposures for each part of the image ahead of time. Don't use Program Mode or Shutter Priority Mode for this. Only Manual or Aperture Priority. The Aperture must remain constant, only the shutter speed can change.
This is much easier to accomplish in the manual mode using your camera's built in spot meter. Set the aperture (and leave it alone). Determine the shutter speed for the needed exposure on the foreground. Remember that shutter speed. Now spot meter the background and set the shutter speed to get the exposure you want for the background.
Put the camera on the tripod and lock in your composition. Take one image at the setting you just determined for the background. Now reset the exposure for the foreground (the shutter speed you memorized from the first reading) and make the next exposure.
Note: this can be done in any order; foreground first or background first. The point is to determine the exposures ahead of time.
Ok, that's it on the camera side. The hard part is remembering you've done it so you don't simply delete two bad exposures. (I sometimes will stick my hand in a picture either before or after the exposure sequence to remind myself I just did something important)
In the computer
Once you get the images downloaded to the computer, open your image editing program. I'm going to base this tutorial on using Photoshop or Photoshop Elements.
Open both images and place them side by side. Click on the Move Tool.
In Photoshop or Photoshop Elements, hold down the Shift Key and click on one of the images with the left mouse button. Hold down the Shift key and the mouse button while you drag the cursor from one image to the other. Once the cursor is over the other image, let go of the mouse button and then let up on the Shift key. What this does is line up the two images pixel by pixel. So if you were using a sturdy tripod and the compositions are exactly the same, everything will be lined up.
Lightroom users have this a little easier. In the Library Module with the image grid showing, select the two images. Right-click on one and select "Open as Layers in Photoshop." This will take care of all alignment.
If you moved images in Photoshop, you may close the image you dragged from if you like.
You should notice in the Layers Palette of the image you dragged on to, that there are now two layers. you can click the "eyeball" icon on the top layer to turn it on and off, just to convince yourself the other picture is still there. One layer contains the picture with the good foreground, the other layer contains the picture with the good background. It doesn't matter which layer is which.
Ok. So now you have one picture on top of the other. Now is where the magic of Layers is unleashed.
Imagine, if you will, that these two pictures are actually photo prints that you have in your hand. You can lay one over the other, hiding the one beneath. Let's say the picture with the good foreground is on top. How can you reveal the good background? A pair of scissors will do the trick. That's essentially what we're going to do now in Photoshop.
You now have a couple of choices in how you reveal the bottom layer. Using a Layer Mask or using the Erase Tool. First, the Erase Tool method.
Click on the top layer to make sure it's active (also make sure the eyeball icon is on so that the layer is visible). Now click on the Erase Tool. Make sure it's at 100% Opacity and that it's a soft brush. Set the size to cover about a quarter of the portion of the top layer that you want to "cut out". You can set brush size, opacity, and hardness or softness from the Options Bar at the top of the Photoshop work are, just below the Menu Bar.
Click on the left mouse button and start erasing the "bad" part of the top layer. See what happens? The good exposure on the bottom layer is revealed. Continue erasing as much as you can without cutting into the "good exposure" portion of the top layer. Go ahead and decrease the size of the brush. (Hint: Use the bracket keys next to the "P" key: [ ] . The left bracket makes brushes smaller, the right bracket makes brushes bigger. You can also change the softness by holding down the Shift Key while using the bracket keys. The left bracket causes the brush to be softer edged while the right bracket makes the brush harder edged.)
As you get closer to the edge of the "good exposure" try changing the Opacity of the Erase brush to around 20% or so and "feather" the transition area little by little with the lower opacity brush.
That's all there is to it! Depending on the image, you may need to zoom in and use smaller and smaller brushes to work around intricate edges.
The other, and probably preferable way, is to use a Layer Mask on the top layer. First click on the top layer to activate it. Then click the Layer Mask Icon in the Layers Palette, it's the one that looks like a rectangle with a circle inside.
Use a black brush to "erase" what you don't want. The cool thing about using a Layer Mask is that if you make a mistake you can fix it simply by painting over your mistake with a white brush, thereby restoring what the mask was hiding.
You can save the image as is, with both layers, or flatten the image by going to the Menu > Layers > Flatten Layers. That's up to you.
See, wasn't that a lot easier than you thought it would be?