Reduce Noise With Dark Slides

Magazine - Digital Photo Pro, Software - Photoshop  Comment
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Some noise is random; some noise is fixed. Hot-pixel noise is fixed. What are “hot pixels”? Photosites on digital sensors that generate brighter information faster than their neighbors. Hot pixels get brighter at higher ISOs, with longer exposures, and in warmer temperatures. You can map where hot pixels are and exactly how bright they get under specific conditions with a dark slide. Then you can use a dark slide to drop out fixed “hot-pixel” noise with a simple postprocessing technique in Photoshop.
To make a dark slide, simply make a separate exposure made at the same ISO, exposure time and temperature as the image you intend to use it with. Exposure of what? Darkness. Leave your lens cap on.
To use a dark slide in Photoshop, open the dark slide and the image you’d like to use it with and drag and drop the dark slide into that image file, holding the Shift key to make sure it’s precisely registered (wait to crop or rotate an image until after this is accomplished). Change the blend mode of the dark-slide layer to Difference and watch the hot-pixel noise vanish.
Learn more about making and using dark slides on Digital Photo Pro.
Learn more about noise here.
Learn more in my digital printing workshops.

Reduce Noise With Multiple Shots

Magazine - Digital Photo Pro, Software - Photoshop  Comment
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Got noise in one exposure? Make a bunch of exposures and watch the noise disappear.
You can reduce noise in an image by combining multiple exposures of the same composition in Photoshop. Photoshop can search for the differences between the separate exposures and then blend them, keeping what stays the same and eliminating what changes. Random noise between separate exposures of the same composition will be substantially, even dramatically, reduced or disappear altogether. (This technique won’t eliminate fixed noise, hot pixels, or column and row noise. There are other techniques for that, like using dark slides.)
You’ll find having this option will greatly reduce any reluctance you have toward using high ISOs. This means two things. You’ll be able to make images in lighting situations you thought you couldn’t, and you’ll be able to make handheld exposures in conditions you ordinarily wouldn’t be able to without severely compromising quality.
So how do you do this? Use the following steps.
1. Shoot multiple exposures.
Try to minimize camera motion as much as possible. It’s not necessary to use a tripod, but it’s helpful2
2. In Photoshop, go to File > Scripts > Load Files into Stack.
3. Click Browse and select the exposures to be used in the Stack and check Attempt to Automatically Align Source Images and Create Smart Object after Loading Layers.
This will create a single Smart Object from the multiple exposures. Double-clicking on this Smart Object will allow you to see the layers separately.
4. Go to Layer > Smart Objects > Image Stack Mode > Median to blend the separate exposures.
You’ll see that the noise is re-duced substantially.
5. Optionally, compare Image Stack Mode > Mean.
This works best for exposures containing no movement.
Read more about this technique at Digital Photo Pro.
Learn more about noise here.
Learn more in my digital printing workshops.