By Jim Hamel
Much of the power of Photoshop comes from its use of layers, which can best be thought of as transparency over your image. Layers have lots of benefits, like the fact that you can work on your image without affecting the pixels in the underlying image. But perhaps the most powerful aspect of layers, is your ability to affect exactly where, how, and how much of your changes will affect the underlying image. That is all done through layer masks. In fact, it might be fair to say that layers are really only effective because of layer masks.
What are layer masks? Think of them as filters that let you control the impact of the changes you make to your image. Just like you can change ordinary filters to determine what gets through them, so you can adjust a layer mask to control what changes. If that sounds confusing, don’t worry, and I will walk you through the basics of layer masks now.
How Layer Masks Work
To understand layer masks, it is best to start with the simplest form. A mask that does nothing – that is, it lets absolutely everything through from the layer you are working on, to the layer underneath – it is represented by solid white. This is referred to as a “reveal all” layer mask in Photoshop. When you look at your layers palette with a white/reveal-all layer mask, it will look like this:
When your mask is all white, any change you make on that layer, will apply to the underlying image. How do you create an all white image layer mask? Very often – as in the case of adjustment layers – one will automatically be created for you. Or, to add one yourself, just go to Layer > Layer Mask > Reveal All. You can also click on this button at the bottom of the layers palette to create one on the layer you have selected
An all-white layer mask is actually pretty useless. The utility of the layer mask comes when we start masking (hiding) things off. Before we get into that, however, let’s talk about how to create an all-black layer mask. As you might expect, a black layer mask works the opposite way as a white one, and lets absolutely nothing through to the layer underneath. You can make all the changes you want to this layer, and it will have no impact on your underlying image. Here is how it will look when you create one:
To create an all-black layer mask, just go to Layer > Layer Mask > Hide All or press ALT while clicking on the same button used in the graphic above to make a white layer mask. Now that you know what layer masks are, and the basics, we can get into the real fun of them, which is where you have blacks, whites, and even tones of gray in the same mask.
Layer masks allow you to apply changes to some parts of the image, but not others. Just remember that wherever your layer mask is white, it will let the changes you made to the layer through, so that they apply to your image below. Wherever your layer mask is black, it will not let the changes through, so they will not apply to the image. Before we get into how to do that, let’s talk about when you might want apply changes to only part of your image.
Let’s take this picture of an old barn. I’m choosing this image because it will be simple to adjust using masks, as it only involves two main areas. As you can see, the sky is too bright, while the barn and foreground are too dark. In doing this for yourself for the first time, use a simple picture like this to get started.
As you probably know, you do not want to edit these two areas in the same way, so a global adjustment to the whole image won’t help. You can, however, use masks so that edits only apply to the areas you want. For example you might darken the sky like this:
Here I masked off the barn and foreground so that the darkening only applies to the sky. I used a quick Curves Adjustment layer to darken the image. As you can see, the mask of the sky is white (so the changes affect the image in that area), while the mask of the barn and foreground is black (so the changes do not affect that part of the image).
Now let’s do the opposite to brighten up the barn and the foreground. Once again, I’ll use a layer mask so that my changes only affect the barn and the foreground, so that it looks like this:
This is the same move I made above, but in reverse. I created another Curves adjustment layer to brighten up the image. Now the layer mask is white over the barn and foreground, which means the brightening effect shows through in those areas. The layer mask is black over the sky though, so the brightening effect does not impact the sky (which is already bright enough).
But how do you do go about creating these layers and masks? Let’s find out now.
Creating a Layer Mask with Defined Edges
There are different ways to mask off portions of your images. Sometimes you will want to create a mask with a hard or definite edge, while other times you will want to gently blend in the effect to a portion of your image. In this article I will cover how to create a mask on an image with a hard edge (I will get into blending in changes in a later article). The photo of the decaying barn used above will work just fine for this, so let’s stick with that photo as our example.
Step 1: Make Your Selection
To darken the sky in the image, the first thing you need to do is select the sky. To do so, click on the Quick Selection tool from your tools panel. Then click anywhere in the sky, and while holding the mouse button down, move around where you want to select pixels for your selection. Photoshop will automatically detect edges. To make the Quick Selection tool larger or smaller, use the square bracket (