If you have taken long exposures you might have noticed annoying characteristics of your camera. I shoot with a Sony DSLR and noticed with my long exposures the camera would take a long time to process an image once I had released the shutter. It was always exactly the same amount of time as the initial shutter speed. For example, if I shot an image for 1 minute it would take an additional minute to process the final result. Total time was exactly 2 minutes. I began to wonder why. I learned that if I wanted to use my camera without the annoying additional processing time, I had to turn off the Noise Reduction. Ok, so I tried that and started shooting again. This time the images were disappointing because the excessive glow that occurred in the corners of the frame. I soon learned that this is the nature of the beast! Either I shoot with Noise Reduction on or learn to reduce amount of noise with shorter exposures.

I wanted to be able to understand what was happening with this noise pattern that was inherit to Digital Photography. I soon learned that it was call Amplifier Glow. This an extreme example of what happens when you shoot and image with the cap on. This is called a dark frame and is a perfect example of a noise signature. The trick that DSLR's employ to get around the noise signature is called Dark Frame extraction. This Dark frame noise signature must be removed from your shot to reduce and hopefully remove your amplifier glow.

We can take these two shots, your original capture and your dark frame into Photoshop to remove the amplifier glow. For this to work satisfactory the two shots must have the same Fstop and same shutter speed. Also because noise patterns are affected by ambient heat the frame should be taken under the same relative time frame. Best to be taken one after the other. In this shot I have applied the dark frame on top of the original shot and changed the layer property to Difference.

This is the result of a manual Dark Frame extraction. So now you know why you might want to have Noise Reduction turned on for your long exposures. It can be much easier for your camera to process the Dark Frame then having to do it manually.

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