Remove Thermal Noise with a Dark Frame Subtraction in Adobe Photoshop

In this episode, I show you how you can deal with the Thermal Noise (Hot Pixels) resulting from a long exposure, with a Dark Frame Subtraction (DFS) using Adobe Photoshop. As mentioned in the last episode (In camera’s Long Exposure Noise Reduction – Good or Bad?) in order to address the Hot Pixel(s) also known as Thermal Noise in your photograph resulting from a long exposure, one can use the functionality found in most modern DSLR which basically follows the Dark Frame Subtraction method. At the end of that episode, I told you, another way of achieving a similar result, if not better, was to apply the Dark Frame Subtraction manually in tools such as Adobe Photoshop.

Two Frames

One still needs to capture two frames using the same equipment, exposure settings and on the same location and under the same climate:

1-The real photograph

2-A dark photograph achieved using the lens cap on

For the experiment here I decided to start from within DPP to show the Hot Pixels given that Camera Raw (the engine behind Adobe Lightroom and Photoshop) deals with Hot Pixel(s) (as much as it can) when interpreting the RAW file, unlike DPP.

From DDP I created 2 TIFF files out of the 2 RAWs and then opened each in Adobe Photoshop.

The Dark Frame Subtraction

In Photoshop, one needs to copy the dark photograph (aka the Dark Frame) onto the real photograph (the first one) as a layer:

Then the trick is simple: select the Dark Frame layer (which needs to be the top layer) and change the blending mode from its default Normal setting to Subtract:

The result is immediate with the Thermal Noise, which manifests itself with the appearance of Hot Pixels, gone:

Before applying the DFS

After applying the DFS

Free alternative tools

While you see me using Photoshop, one does not need to purchase any software to be able to apply this method. Here is a couple of links to free tools found on the market.

GIMP

BlackFrame (Shareware)

Conclusion

This manual DFS approach is not quicker than the In Camera’s treatment of course since one still needs to take two exposures. As a matter of fact, this manual approach will take you longer since you need to apply the method yourself. However, there is one exception to that “it’s not quicker”. Should you be taking several expositions with the same parameters, then you would not need to take several dark frames. One would suffice. So in this particular case, it would be quicker then enabling the option in your camera for every take. Star trail photography using the stacking approach is a good example. I then recommend making the dark frame the last exposure you take as your sensor would heat up over time especially if you were to take hundreds of frames in the scenario of star trails. In general, I would also add that while some may call me old fashion, I’d like to think the result is better when performed in the software as the tools are smarter than the mini computer inside the camera. However that being said, the DFS is a simple subtraction of the real photograph by the dark frame, so it is not rocket science either.

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