Use Smart Object Stack Modes in Photoshop CC
Working with Photoshop CC, you can combine a number of images into a single stack as a Smart Object. Within the pile of images, you can determine how the pixels in each interact with those in the others.
Select several related or contrasting images and add them as layers to a single image, select the layers, and create a Smart Object by choosing Layer→Smart Objects→Convert to Smart Object.
Return to the Layer→Smart Object submenu and take a look at the Stack Modes submenu that’s now available. These options determine how the content on the layers within the Smart Object interact to produce the appearance of the Smart Object itself. (Not quite the same, but similar to the way layer-blending modes help determine the overall appearance of your artwork.)
You can find technical explanations of each option in Photoshop's Help, but here are a couple of stacks as examples. On the left in this figure, you see a combination of the layers (shown to the left as thumbnails) using the stack mode Summation, which pretty much adds up all the lightness of each pixel in each channel, on each layer.
To the right, several images shot for use with Merge to HDR are combined in a Smart Object using the stack mode Mean, which averages the values for each pixel in each channel. Not quite Merge to HDR, but with some planning and prep, it could be a supplemental technique.
Photographers may also find another great use for stack modes. Using a tripod and the same exposure settings, take a number of photos of a static object with moving objects (people or cars, for example) over a reasonably short period of time (minutes, not hours, so the lighting doesn’t change much).
Select the thumbnails in Bridge and use Bridge’s menu command Tools→Photoshop→Load Files into Photoshop Layers. (If you’re not using Bridge, open the first image, then open each additional image and drag the layer Background from the Layers panel to the first image.)
In Photoshop, use the menu command Select→All Layers. (You never even noticed that command before? Now you’ll use it all the time.) Unless you had a rock-solid tripod, choose Photoshop’s Edit→ Auto-Align Layers command with the Reposition option.
Create a Smart Object from the layers by choosing Layers→Smart Objects→Convert to Smart Object. Choose the Median stack mode and watch all of those moving objects simply disappear, leaving you with just the scene, empty of traffic. This figure shows the shots originally taken and the resulting image.
If something very prominent or bright in one of the shots doesn’t disappear completely, go back in the History panel to before you created the Smart Object (the Auto-Align Layers step) and delete that particular area from the offending layer, re-create the Smart Object, and reselect the Median stack mode.
One other stack mode of value to photographers is Mean. Shooting in low light with a high ISO setting can result in images with a lot of noise. Using a tripod or bracing the camera, take several shots. (This is a great trick for shooting in a museum that doesn’t permit flash photography.)
Open the files into Photoshop layers, select all the layers, and align. Create your Smart Object from the shots and set the stack mode to Mean to minimize the noise.