The human eye can perceive a contrast ratio of 800:1; the best slide films, 30:1; and the best digital sensors, 40:1. So photography has always been about compromising. When shooting slide films, the old adage was “expose for the highlights, pray for the shadows.” With digital, that has changed a bit, especially with RAW files.

You can process the RAW file two (or more) different ways and combine these images using an image-editing program to get the best of each exposure. When shooting with a digital camera in the RAW format, you have the capability of processing your image specifically for different exposure values, highlights, midtones and shadows. You can use this flexibility of the RAW format to your advantage when photographing scenes with a high dynamic range of light.

Is it best to slightly underexpose or overexpose RAW images? When you lighten a dark RAW image, you risk adding a lot of noise to the shadow areas. When you darken a light RAW image, you might not have good highlight detail. I find when shooting under contrasty light that it helps to slightly overexpose the image and then process it twice—once to have more shadow detail and once to bring back the highlight detail.

The image shown here is from the wonderful light show at Chicago O’Hare International Airport in the tunnels between terminals. I had a four-hour layover at the airport, and being a photographer, I appreciated the neon lights that played around the moving walkway that connects the different concourses, so I decided to photograph them. I rode back and forth on the walkway, shooting many different exposures at many different shutter speeds.

That’s one great thing about digital—using the image review on the LCD, I could check the shots and look for such things as the highlight warning to make sure I wasn’t overexposing the bright highlights. I placed the highlights just on the verge of overexposing so that I could keep the overall photo bright while still retaining detail in the highlights.

When I returned home, I processed the image in Adobe Camera Raw, built into Photoshop CS2. Photoshop CS2 is the first version of Photoshop that allows you to open two copies of a file with the same name, which makes this technique easier. Otherwise, when you first open a processed file, you have to rename it before you can open a newly processed image from the same file.

(STEP 1) Process For Highlights. The first time I processed the image, I moved Exposure down to a minus number to bring the overall exposure down about a half-stop. I watched the bright colors to make sure I held all the detail and color in the brightest areas of the picture. The darkest parts of the photo are unimportant at this point. I opened this version into Photoshop.

(STEP 2) Process For Shadows. I went back into Camera Raw and processed the image a second time. This time, I opened up the shadows by moving the Shadow slider to the left (which reduces its setting) and by adjusting the Exposure slider to the right. This processing step is purely about the dark areas, so having the bright colors lose detail didn’t matter. You may find, as I did, that when the shadows are opened up like this, they appear a bit flat. I added a little contrast to the image because of that. I also opened this version into Photoshop.

(STEP 3) Create A Layered Image. Now you bring the two processed images together into one file. Since they come from identical files, they will match exactly. I put both images side by side in Photoshop. Using the Move tool and holding down the Shift key, I clicked on the lighter image and dragged it in perfect registration onto the darker image (the Shift key keeps them aligned). You must move your cursor completely onto the second photo or you’ll get an error message that says you can’t move the background layer.

The result is a file with two pixel layers, the light image over the darker image. You’ll find that some photos do better with the darker image over the lighter image. There’s no absolute rule for this, though often it’s best to put the image needing the least amount of work on top.

(STEP 4) Add A Layer Mask. I then added a layer mask to the lighter image layer. The easiest way to do this is to click on the Layer Mask icon on the Layers palette (the icon is a rectangle with a circle inside it). This creates a Reveal All (white) layer mask on the active layer (you also can go to Layer > Layer Mask > Reveal All). The secret to all masks in Photoshop is that black hides the layer and white shows the layer. At this point, all the pixels of the lighter layer show because the layer mask is white.

(STEP 5) Bring In The Lower Layer. To show the darker pixels of the layer below, you need to paint with black on the mask to hide the pixels of the lighter layer. In this case, I needed to balance the bright lights along the top of the photo with the darker bottom area of the image and I also needed to bring in the better highlights of the underlying layer to blend into the upper layer that had the good dark tonality. I used the Paintbrush with black as the foreground color in order to paint with black on the upper portion of the mask.

With the layer mask active (click on the mask to make sure it’s active), I selected a soft brush with a fairly low opacity; I recommend 20% to start. Then I painted over the image. I built up density by going over some areas multiple times. If you just drag your cursor back and forth across the image, you won’t be able to build up any more density than the 20% opacity you set. By releasing the mouse or raising the pen (on a tablet) as you go, you can build up density. If you use a soft brush and build up the density of your strokes, it’s easy to make the blends seamless.

(STEP 6) Refine Your Layer Mask. You can see your mask by Alt/Option-clicking on the Layer Mask icon. From the screenshot shown here, you can see that the edges of the mask were refined and blended to make the two photos go together better.

Once the mask was complete, I had an image that showed good detail and contrast throughout the image, balancing the exposure. So what was once difficult to record in a single exposure is now possible.

With one click of the shutter, you can record the information needed in a RAW file, which you can process in different ways to bring out better detail in both the highlights and shadows. Then it’s a simple matter of combining the two images by using a layer mask to make your picture more closely resemble what your eye could see instead of what technology can record. It opens up many possibilities for capturing reality in a way that was never before possible.