What is Dark Frame Extraction?
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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March 11, 2008 7:22 AM
Hi, Rob. It's been a long time I wasn't able to visit my blogroll. I had been very busy this past few months.
Looks like this one is a very interesting post. I, however, ran out of time today. I'll visit here again tomorrow to read all of these stuff here ;)
March 18, 2008 4:57 AM
Hi, Rob. that was quite an observation... I also actually noticed that - that when you use long exposure, the processing of the image also would take a while. But I didn't know it was because of the noise reduction processing. I'm using an old Canon SLR so there's no option to turn off the noise reduction. I also got a noise when using higher ISO (of which ISO3200 is the highest setting).
But for me, I'd rather choose that noise reduction be done "inside the box" than to be "Photoshopped" later. While the later is more technical, that will consume much of our time. As you say it, "It can be much easier for your camera to process the Dark Frame then having to do it manually."
I also wish you can run again. I also would like to do such exercise but don't have the appropriate time and place...
I also would like to congratulate you for your pointing of your blogspot to a subdomain of your own domain name. I tried myself doing it but there was no lack. I guess it is my hosting service (no option for CNAME changes). So, I have had all my hosting in paid hosting service...
This week is already Holy Week here. I have no idea where to go when I break from work. What about you?
March 23, 2008 1:07 AM
Hi, Rob. Happy Easter to you, my friend...