Layers and Layer Mask

The layers are one of the most powerful and flexible tools of Photoshop: I use extensivly layers for selective adjustments of color, contrast, noise reduction, ect, and for particular techniques as blended exposures. The layers are like paper sheets placed one upon the other. You can hide part of the content of a layer, or make it semi-transparent to see the content of the layer below. When you open a photo, there is only one layer (the "background") that contains the image. You can copy the entire image, part of the image, another photo or everything else on a second layer. You can create as many layers as you want, you can delete layers and you can adjust selectively the properties of every single layer.

The Layer Mask

The layers mask allows to show only part of the layer, using a simple mask and the PaintBrush Tool. If you have never used the layer mask, you can learn how it works in few steps: it is much more straightforward than what you may think. Open an image with Photoshop and paste another image into a second layer.

Example image one (Layer 1)	Example image two (Layer 2)

Select the layer "Layer 2", and click on the menu Layer > Add Layer Mask > Hide All to apply the layer mask. The Layer 2 becomes invisible, and a black box (that represents the mask) appears near the layer icon. Select the PaintBrush tool: be sure that the color of the brush is set on white. Now, "paint" on the image: the areas that you paint are displayed in white into the small mask icon, and they reveal the masked image.

What is it happened? With the PaintBrush, you have deleted part of the mask the hides the Layer 2. 

Now, you are using a white brush. Select the color black and paint again on the image: the areas that were revealed become again invisible. The white brush erases parts of the mask; the black brush re-build the erased areas. Re-select the color white, paint on the image and observe the alteration into the mask icon. Repeat the process until you firmly understand how the layer mask works.

The next paragraphs illustrates some of the many applications of the Layer Mask.

Blended exposures to expand the dynamic range

With layers and layer mask, you can blend two or more photos to create a single image with extremely wide dynamic range: I often use this technique for sunrise and sunsents, it gives spectacular results! The majority of DSLRs has a dynamic range of 7-8 stops:  even though it is very good, in some situations it is not enough. Blending exposures allows to capture even the most difficult scenes, without limits of dynamic range.

The first step of the technique consists in taking multiple exposures of your subject. Place the camera on the tripod and take one or more photos of the same scene varying the shutter speed; one photo should be exposed for the brighter areas of the scene and the other frame for the darker areas. Use the histogram to check the exposure.

The photo exposed for the sky	The photo exposed for the foreground

The first photo is exposed for the brightest areas (the Sun and the sky), while the second photo is exposed for the darker areas (the foreground). The camera was mounted on a tripod and the parameters (aperture, white balance, iso, etc..) were the same for every image; I changed only the shutter speed.

Open the RAW files with Adobe Camera RAW with the same parameters; do not apply any exposure correction. Use the image exposed for the dark areas as background, and place the other image into a second layer.  With the layer mask, you can reveal only the correctly exposed areas of each image. A large (500, 1000px or more) and soft brush is ideal to create a smooth and natural transition between areas of different brightness. You can use small brushes to make the fine corrections,

and the black brush to hide the areas that had been accidentally revealed.

Selective sharpening

When the photo has large out of focus areas, I always use the Layer Mask to apply sharpening only on the subject. This technique is particularly useful for photos taken at high ISO, both for full size files and web-sized images.

Sharpening on the entire photo	Sharpening only on the subjecy

If you apply the sharpening on the entire photo, the noise into background becomes clearly visible. To get the best results, instead, I create a copy of the photo on a second layer (right-click on the layer palette and select "duplicate layer" from the drop-down menu). I apply the Smart Sharpen on the copy, while I apply a strong noise reduction on the background, than I apply the layer mask on the copy. With the paintbrush, I reveal only the sharp areas of the photo (the subject): the result is a razor sharp subject against a smooth, noise free background.

Other selective adjustments

Usually, the adjustments of contrast, saturation, color, etc. are applied on the entire image, but in some cases you may prefer to apply them only on a particular area. With the layer mask, you can brighten up only a particular area of the photo (for example the shadows in a landscape); you can increase the contrast only where you want to give more punch to the image (e.g. on the wings of a butterfly), you can exaggerate the effects of light....the only limit is you creativity!

The two photos above are an example of selective adjustments with layer mask. Here, I wanted to increase the contrast between the warm light of sunset and the blue tones of the shadows. I've made a copy of the image on a second layer, then I increased the contrast with an S curve, and I warmed up the color with Color Balance. I applied the Layer Mask "Hide All" and I used the white brush to apply the effect only on the sunlit areas, without affecting the shadows, that retained their natural blue cast.

Do you have comments or questions?

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