Last week we took a look at Stack Modes in Photoshop, and found that all we needed was a layered Smart Object, and we could use the Stack Modes to great effect. This week we will continue to look at uses for Smart Object Stack Modes, looking at a very effective technique for removing color noise in photographs. I first saw this demonstrated by Katrin Eismann at Photoshop World some years back, when CS3’s stack modes were first introduced as part of Photoshop “Extended.”
You are surely familiar with color noise – the downside of digital photography, which is brought on by either inexpensive sensors or high ISO settings. It appears as a splattering of magenta and green spots throughout your image, particularly in the shadows, and brightening the shadows always seems to bring the color noise out even worse. For example, here is an image taken at ISO 3200:
Not so bad, you think? Lets zoom in and take a look at 100%:
See the purple and green “spattering” going on? That’s color noise. Color noise is caused by random fluctuations in the receptors of our camera sensors, and turning up the “volume” (high ISO) turns up the noise level along with the sensor’s sensitivity. However, the key to solving the noise problem with Photoshop Stack Modes, is the fact that the noise is random. It is never the same, even from image to image. We can take advantage of that.
If we take a series of pictures of the same scene, and align them by either using a tripod (preferred) or using Photoshop’s “auto-align layers” feature, we can take advantage of the fact that because the noise is random, those green spots and purple spots will NOT be in the same location on multiple images. We’ll use Photoshop’s Stack Modes to make the noise go away!
For this example, we’ll start in Adobe Photoshop Lightroom. Highlight all the images you’ve taken, and right click (command-click on a Mac) and from the context menu, choose Edit In > Open as Layers in Photoshop…
Note: If you’re working in Bridge, you can highlight the group of files and use Tools > Photoshop > Load Files Into Photoshop Layers…
Once you’re in Photoshop, we will follow the same process we used last week in processing our layers for Smart Object stacks:
Shift click on all the layers to select them all, then choose Edit > Auto-Align Layers…
Use the defaults and choose OK:
From the Layers Panel fly-out menu (with all the layers still selected), choose Convert To Smart Object:
Now that we have our Smart Object, we’re ready to apply a stack mode. The best stack mode for noise reduction is “Median,” which will calculate each pixel’s level between al the images and minimize any variance (if you have five images, and only one of the five has noise in a given pixel, the result will be a color much closer to the remaining four images).
Choose Layer > Smart Objects > Stack Modes > Median:
After a moment or two (or several, depending on the size of the image and the horsepower of your computer), Photoshop indicates that a Stack Mode is applied with an icon in the Layers Panel:
Here’s the resulting image:
Now, let’s compare a 100% zoom close-up of the final image with the first image:
As you can see, quite a difference in the color noise! Smart Object Stack Mode, set to Median, has eliminated the random noise that was ruining our image. All we had to do was combine multiple images together as a Smart Object.
Will this technique work every time? No, this only works when you have the chance to capture 3-5 images or more of the same subject, with no movement. For still night scenes, this technique can work wonders.
Next week, we’ll look at another useful application of Stack Modes – the “automatic tourist remover.” I hope to see you then!
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