Why Subtract Dark Frames?

I’ve seen many people doing night time / astrophotography I thought it would be beneficial if I did a really quick post on Dark Frame Subtraction.

Below are two images, one shows a section of a photograph before dark frame subtraction and the other shows after. (note the red dot in the before image)

Without Dark Frame Subtraction
Red Dot is Generated From Camera
With Dark Frame Subtraction
Note Red Dot is Gone!

First I’ll explain what a Dark Frame is for those who do not know. A dark frame is simply a photograph taken in complete darkness that is designed to capture the “Bad Pixels” on your camera sensor. When you take long exposures your camera tends to get “Hot Pixels” which show up in your image (sometimes as red dots). The intent of Dark Frame Subtraction is to reduce the amount of noise that is produced by your camera. 

Many of today’s cameras have “High ISO” or “Long Exposure” noise reduction. Essentially this does the exact same thing that I’ve done in post processing for this example. To explain this a little more, when you use these settings your camera will take the “Light Frame” (image that you are photographing), and then take the dark frame (image with the mirror down), it then subtracts the dark frame from the light frame, leaving you with the final image. That is why (you may have noticed) that it takes about twice as long for your camera to show the preview image when using these settings. Processing your images this way is just fine if you are taking a single exposure, but what if you are doing star trails, or even Deep Space Photography where several images are combined to give you the finished product. Star trails will have gaps and appear as a series of “Dashes” if in camera noise reduction is used. That is why it is important to learn how to do this in post processing.

Making your dark frame is easy, but there are a few things that you have to remember. Number 1 – Take your dark frame with the EXACT same settings as you did your picture (Noise changes with different camera settings). Number 2 – Take your dark frame at the same ambient temperature as you did your light frame(s) (outside temp). Temperature also affects noise.

How did I do this
- Open your two images in photoshop (or elements)
- Now that the two images are open, go to the dark frame and select the whole dark frame by using the “Marquee Tool”
- Copy the image
- Paste the dark frame on top of the Light Frame
- Using the layer mixing options select “difference” (here you can play around with the “Opacity and Fill” levels to suit your taste.
- Once you have achieved your desired results flatten the image and voila, you’re done.

Important note
There are many programs out there today that you can use to do this automatically (using your dark frame) when combining deep space photographs or combining images for star trails. It is usually much easier to just use these programs, but it is always nice (and important) if you understand why you are doing something, or how it is actually being done.

Clear Skies!
Darryl W. Van Gaal