AnandTech Guide to Better Photos: Night Photographyby Stephen Caston on February 18, 2005 12:05 AM EST
- Posted in
- Digital Camera
Take Two: Removing Hot PixelsOne of the problems that you may run into when shooting long exposures is the appearance of “hot pixels” in your images. These are pixels on the image sensor that suffer from charge leakage after being exposed to light for a certain amount of time. The effect is bright out-of-place pixels appearing throughout your images. You may notice that these hot pixels are appearing in the same places in each of your long exposures. Although this can be frustrating, there is something you can do about it. In fact, the consistency of the location of the hot pixels is beneficial because it allows us to use a method called “dark frame subtraction” in order to remove them. Some cameras will perform a dark frame subtraction automatically, but we are going to show you how to do it manually.
Immediately after taking your normal long exposure frame, cover your lens and take an additional “dark” frame with the same exposure settings. A change in temperature may result in more or less hot pixels (higher temperatures result in more), so be sure to take the dark frame right after your original image. Taking the dark frame with the same exposure settings will be easiest if your camera has a manual exposure mode. Otherwise, you can try to lock the exposure before covering the lens. In some cameras, this can be done by half-pressing (and holding) the shutter button or pressing the “AE Lock” button. The idea here is that the dark frame will be used later as a map of all the hot pixels. This makes it very easy to remove them later. Now that you’ve taken your original image and a dark frame, let’s look at some ways to remove those dreadful hot pixels.
NOTE: For this section, we will be referring to steps that can be used in either Photoshop or Photoshop Elements. Photoshop is the industry standard for graphics design and likewise, it carries a hefty price tag. Photoshop Elements is a simplified version of Photoshop that offers many of the same useful features at a fraction of the cost.
- Open your original image and your dark frame.
- Select the “Move” tool from the tools palette (or press “v”).
- Click and drag the dark frame over into your original image while holding down the Shift key. By holding Shift, the dark frame will center itself automatically on top of your original image. This is important so that the hot pixels will be aligned perfectly in both images.
- On the layers palette, change the blend mode of the dark frame layer to “Difference”. At this point, you should no longer see the hot pixels in your image. You can adjust the Opacity of the dark frame layer until the image looks good to you.
|Dark Frame Subtraction
Dark frame at 100% opacity
Dark frame at 50% opacity
In the 200% crops above, we have demonstrated a dark frame subtraction. The original image suffers from a high frequency of blue and red blobs. In the second crop, we added the dark frame layer and set the blend mode to “Difference”. With the opacity at 100%, you can see that the blue and red blobs are gone. However, in their place are dark pits and greenish blobs. By setting the opacity to 50%, we were able to find a good balance to produce a much cleaner image. As we stated before, temperature plays a large role in the amount of hot pixels that will occur. Even so, each sensor will be different and consumer digicams tend to suffer from hot pixels more than prosumer and professional models.