Blending Images to Control Dynamic Range

Photoshop has a Highlights/Shadow tool that can be used to control tonal values in high-contrast scenes. However, using this tool can often leave the picture looking a bit “strange”. We take a look at creating a nicely exposed pictures by blending two images, one exposed for highlights and one exposed for shadows. Note that this is NOT an HDR process, eventhough there is some resemblance.

Controlling Dynamic Range

I shot two pictures of the same scene, but each with a different exposure. By using the histogram, I ensured that I captured all the highlights in one image and all the shadows in the other.  Here are the two pictures with their corresponding histograms:

20060908-img_2630

Overexposed, but shadows OK

Clipped Highlights

Clipped Highlights

We can clearly see how the first exposure has clipped highlights (shown by the red circle). In other words, critical tonal information falls outside the tonal range of the picture. That data is lost and we cannot retrieve it. In the picture this shows up as a white expanse in the sky. No detail in the clouds, just a white patch.

The rest of the picture is very usable. We see detail in the shadows and the histogram tells us that all the dark pixels fall within the tonal range of the image.

Underexposed, but highlights preserved

Underexposed, but highlights preserved

Clipped Shadows

Clipped Shadows

In the second image we see just the opposite. There are dark pixels that have been clipped, as indicated by the red circle in the histogram. The picture shows black areas with no detail, just a black patch. The sky however is properly exposed. We can see detail in the clouds and the histogram tells us that all the light pixels fall within the tonal range of the image.

After opening both images in Photoshop we can combine these two images and blend them using the properly exposed portions of each shot to makle a composite that is nicely exposed for both shadows and highlights. The first thing we need to do is combine the two shots. We’re using Photoshop so you can do this simply by opening both images and dragging the lighter exposure onto the darker one. You can do this by selecting the layer and just dragging it to the darker image as seen in the screencapture below.

Drag one layer to the other image

Drag one layer to the other image

Hold down the “shift” key when you do so. That will cause the new layer to align with the underlying layer automatically.

You should now have two layers. Your layers palette should look like this:

Two layers

Two layers

With the new layer selected (the top one), pull down the “Select” menu and select “Color Range”

Select Color Range

Select Color Range

In the Color Range Dialog box, make sure you select “Highlights” and tick the “Invert” option.

Explanation: The term “Color Range” is somewhat misleading here. By picking the “Highlights” option, we are asking Photoshop to find and select all pixels that are considered to be highlights. This is far more accurate than us trying to select a portion of the image manually. Especially in shots with intricate detail such as tree branches, etc. Photoshop can do a much better job.

We are going to use this selection to create a mask. A mask, in Photoshop, lets us selectively hide or expose portions of underlying layers. Remember we have a darker image underneath the one we’re using and we want to hide the overexposed portions of the selected layer (which are the blown areas in the sky) and have the properly exposed sky from the underlying layer shine through instead.

The darker a mask is, the more of the lower layer shines through. Therefore we need to make the selected highlights dark and we do this by inverting the selection. See the screenshot below, the sky is black, the rest is white. Now click OK. You should see your image with sections selected with the so-called “marching ants”

Highlights are selected

Highlights are selected

Now we can create a mask using this selection. We do so by clicking the Add Layer icon in the Layers Palette.

Click Create Mask Icon

Click Create Mask Icon

When you do that, your picture will look quite awful! That is because the transition between the two layers is too abrupt. You fix this by blurring the layer mask. Do so by selecting the mask and go to “Filters” – “Blur” – “Gaussian Blur”. Slide the slider to the right until you like the result. In this example, we needed to go all the way to 250 pixels.

Blur the mask

Blur the mask

Now you should have a nicely blended image with highlights and shadows perfectly exposed and you can apply further enhancement techniques to finish off the image. See below the properly exposed image, ready for color correction and other processing steps.

Blended image with perfect exposure

Blended image with perfect exposure

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