Dark Frame Subtraction Tutorial - Fix Amp Glow!

August 01, 2017  •  Leave a Comment

One of the most frequent and annoying problems I face when doing long exposures at night is amp glow.  Amp glow is caused by two main factors: not capturing enough light and sensor heat.  The more light you capture, and the cooler the camera sensor is, the less noticeable amp glow will be.

A lens with heavy vignette will show more amp glow than a lens with minimal vignette.  Since less light is able to reach the corners, noise is exaggerated when the exposure or shadows are raised in post-processing.  By capturing more light, this noise is 'covered up' by light.

The Rokinon 14mm has an awful vignette, which prevents light from reaching the corners of the sensor.  The Nikon 14-24mm on the other hand, has a minor vignette; more light is able to reach the sensor.  As you can see in the comparison below, the Amp Glow is more exaggerated in the Rokinon photo.

 

Remove Amp Glow - Dark Frame Subtraction

The preferred method of removing Amp Glow is called a Dark Frame Subtraction.  As soon as you take the normal photo, put the lens cap on the camera and take another photo.  All the settings should stay the same.  This Dark Frame will capture the noise and heat levels on the sensor, as no light is actually reaching the sensor.  Once you have a Dark Frame, you can use it in Adobe Photoshop to remove the Amp Glow.

I have a few important points to make before we continue.

  1. Turn on Long Exposure Noise Reduction for both the normal photo and Dark Frame
    • This will remove Hot Pixels and make things easier later
  2. Do the exact same edits to the Dark Frame as to the normal photo
    • If both photos aren't edited exactly the same, you will have problems
  3. Increase the Luminance Noise Reduction in Adobe Camera RAW
    • Otherwise you will see millions of black specks - the noise in the dark frame
    • The black pixels disappear when the Noise Reduction is done properly


The photos below show how well the dark frame subtraction worked on three lenses: Nikon 14-24mm, Tamron 15-30mm, and Rokinon 14mm.  Every time I've tried doing a dark frame subtraction on the Rokinon, the image has always become too green.  I was happy to see the process finally worked as intended on the Nikon and Tamron.  

 

Remove Amp Glow - Longer Shutter Speed

Alternatively, you can simply take a longer photo to help remove Amp Glow and noise!  If you are photographing the Milky Way on a moonless night, try taking a 4 minute long photo at ISO 800.  This should capture enough light to "cover up" any Amp Glow.  However, the sensor will likely get very hot over the course of that 4 minute exposure.  This will cause Hot Pixels to appear all over the image.  I recommend turning on Long Exposure Noise Reduction beforehand.  This will take a second 4 minute exposure, and automatically remove the hot pixels.  Of course, you now have to wait 8 minutes total for 1 photo!

Keep in mind, the stars will move significantly over the course of 4 minutes.  If you do this method, you will need to take a normal photo for the sky and blend the two exposures in post-processing.  The images below show the difference between a normal, 20 second photo, and a 4 minute photo.  Notice how much less noise and Amp Glow is visible in the longer exposure.  It's incredible!  

*Note: Both images have been brightened in Adobe Camera RAW to better show the foreground.  The RAW photos were much darker straight out of camera.

You can click each image to view the full-res photo and download them, if you want a closer look.

20 Second Exposure20 Second Exposure 4 Minute Exposure4 Minute Exposure

 

Tutorial Video

I created this video to clear up any lingering confusion on this process.  If you have any questions, feel free to leave a comment and I'll do my best to help you out!

 


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