Using Luminosity Masks to Merge Photos

One of the biggest challenges in photography is dealing with high dynamic range. Simply put, our camera is not capable of capturing the full range of brightness to darkness that we see with our eyes. There are several ways that we can deal with high dynamic range.

If the issue is a bright sky, we can try am make use of graduated neutral density filters. In some situations, these work very well and allow us to capture the image in one shot. The problem is, many times the delineation between the sky and the darker parts of the image are not anywhere near a straight line. When this happens, we may get lucky and have enough detail in the shadows caused by the filter to blend the darker areas with the rest of the image. This can be a very time consuming activity, and if we are not careful can lead to those annoying halos.

The next tool we have available to us is HDR software. Here we take several photos with different exposures and combine them using a software application like MacPhun's Aurora. When done properly, these images can look very good. When not done properly, they tend to be rather garish. Sometimes, for whatever reason, it seems that it is impossible to get the results that are desired from HDR.

Luminosity masks have been around for a long time. I think many people avoid using them for two reasons:

Setting up luminosity masks takes a little bit of effort.
Many people are intimidated by masks.

Let's look at these two objections. While the first is certainly true, there is a very simple solution. Many photographers have created a set of Luminosity Mask actions that you can download and install for free. Simply Google free luminosity mask actions and you will be presented with dozens of websites. Download a set and install them.

The sites typically provide the instructions on installing actions into Photoshop if you are unaware of how to do that.

The second objection, I think, has a lot to do with how difficult it can be to select an area of a photo and make a mask. This is where luminosity masks make things easy. The mask gets created for you based upon the brightness that is selected.

Let's look at an example. I came across this stream in northern Minnesota. Given the time of day, I knew that I could not capture it in a single image so I went about collecting a set of images for HDR:

The problem was that after running it through my HDR software and working on the image in Lightroom and Photoshop afterwards, I could not get the look I wanted. Here is the result:

This definitely has that HDR look and feel. Clearly, I needed to dig back into my tool box and find a better way to create this photo. This is where luminosity masks help.

I will begin by picking the image with the right level of exposure in the grasses and trees (image #1 in the first illustration). I then select the image with the right level of sky exposure. I chose image #3 even though the histogram was telling me that there are a couple of spots that are blown out. With the two images selected, right click and select Edit In/Open As Layers In Photoshop.

This will cause Photoshop to open and the two images will appear as layers. I prefer to have the brighter image on the bottom to use as my base or reference image. If they show up in the opposite order, you can drag the image under the darker one.

Make sure both images are selected. We need to align the layers just in case there was some camera movement. In my case, I did slightly bump the camera between shots. To do this, go to the Edit Menu and select Auto-Align Layers.

With the layers aligned, we can now create add a mask to the darker image. Ensure that the mask is set to black. This will allow the brighter image to be displayed since the darker image will be all masked out.

Select the base image. This is the image that we want to use to create our mask. With the base image selected, run the luminosity mask actions that you installed.

Once the action is run, select the channels tab. You will see a list of all the masks available. Pick the level that matches what you want to replace. In this example, we are going to use Brights 2. To select it, hold down the command key (control if you are on a PC), and click on the level.

Once the level is selected, the "marching ants" will be displayed showing everywhere the mask is applied. If you look closely, you will notice that the mask does a good job of matching the edges. All that is left is to select the mask that we added earlier. Paint "white" over the parts of the image where we want to the darker exposure to show through. The cool thing is that we don't have to worry about "coloring inside the lines."

Once all the areas are painted in, all that is left to do is flatten and save the image. Once the image is saved, it will then appear back in the Lightroom catalog.