Luminosity Masks – Why Can’t I Paint Inside The Selection?

As beginners to luminosity masks, there are a number of challenges we face in understanding their application. LMs are a wonderful tool for any landscape or cityscape photographer, but one of the most common questions I’m asked by newcomers to LMs is, ‘why can’t I paint inside the mask selection?’

It is easy to understand why we first come up against this challenge. When I ask these photographers to show me the images they’re working on, I often see the same type of scene, similar to the one below.

When I say the ‘same scene’, I’m not referring to cityscapes, but to the light distribution of the scene – in other words, how much contrast is in it.

The scene above is flat, with no over or under exposed areas. Often, with scenes like this, people will try to paint in the sky from a darker exposure, without wanting to affect the buildings. Their goal, therefore, is to use luminosity masks to make a selection of the sky and not the buildings.

However, with luminosity masks this isn’t possible because the buildings and sky are of a similar brightness. But more importantly, it isn’t necessary to combine exposures here. The goal of exposure blending is to recover over exposed or under exposed areas. If your image doesn’t have any, then you are in a very fortunate position. You can do all of your editing on a single image, which is more straightforward.

In fact, much of your editing can be done in ACR or Lightroom. You can bring back some of the highlights with the Highlights slider, and recover the shadows with the Shadows slider. Then you’ll have an image with the full range of light.

Let’s take a look at an example luminosity mask from this image. Below is a Brights 2, one of the masks we’d typically use to recover a blown out sky.

As you can see, the top of the buildings are grey, as is the sky. If we try to create a mask here, both the sky and buildings will be selected because there is not enough difference in brightness between the two objects. There will be some separation, because the buildings are marginally darker than the sky, but the difference isn’t strong enough.

Let’s look at a different example where luminosity masks will work. Below we have a seascape where the sky is over exposed. We also have a sea stack in the background and cliffs to the right. We want to paint the sky from a darker exposure into this exposure, but without affecting the sea stack and cliffs.

After running luminosity masks, this is what Brights 2 looks like:

In the mask the sea stack is almost completely black, while the sky is either white or light grey, and parts of the over exposed water are also included in the selection. This is almost a perfect selection. By using this mask, we can paint the sky in from a darker exposure, while not affecting the sea stack and cliffs.

This is what a quick paint using Brights 2 with a white brush at 100% opacity achieved (no other processing applied).

 

To Finish

As you can see, it is essential that you have enough contrast between the areas you wish to select and the areas you don’t wish to include in your selection in order to paint within the mask.

But with scenes of low contrast, ask yourself first, ‘do I need to blend multiple exposures or will 1 exposure suffice?’ If there are no clipped areas, stick with one exposure.