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09-26-2009, 03:21 PM   #1
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Dark Frame Subtraction?

What exactly is Dark Frame Subtraction? How and when do you turn it off?

From what little I could get from my searching, it is something you want to turn off in the camera for long exposures and has to do with reducing noise (but I could be off here)

Several questions come to mind:

At what point is an exposure considered "long" --1 sec, 2 sec, 3 minutes?
Why exactly do you want to turn it off during a long exposure? Does it negatively impact the quality of the photo? What if you leave it on?

I have both the K100D and the K200D. I think I can turn off DFS...assuming I knew how to do it and why I should

thanks!
sue

09-26-2009, 03:47 PM   #2
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It's called "slow shutter speed NR" or something like that in the custom menu; that's where you'd turn it on or off. When on, it kicks in for exposures of longer than a second or two, I think.

What it does is take a second picture after the one you really took - the second one with the shutter closed (hence a "dark frame"). So you press the shutter button for a 5-second exposure, the camera spend 5 seconds takes a picture, then closes the shutter and spends 5 mores seconds taking a second exposure with the shutter closed. The idea is to see where that second pictures shows noise or hot pixels, which is really easy to do since it *should* be completely black - any pixel readng that isn't 0 is noise. Then the camera compares that pictures to the real one and uses the second as a guide to remove the noise and hot pixels in the first (I think it literally goes pixel by pixel subtracting the value in the second picture form the value in the first).

Without this, night sky pictures - ones in which most of the picture should be black - tend to look terrible. But sometimes certin scenes can look OK without it, so the option to turn it off is appreciated by many. Doesn't seem a big deal when talking about 5 seconds, but take a 30 second exposure, and having to wait *another* seconds for the dark frame is kind of a pain.
09-26-2009, 04:18 PM   #3
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The things you learn in this forum. that explains the ten minute wait after my night shots last night. thanks for asking and thanks for answering
10-01-2009, 12:55 AM   #4
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If you take multiple equally long exposures of the night sky for example, and the camera has been in the same weather conditions, you could turn the dark frame function off and take your own dark frame at the end just by covering the lens. Later on you can use that frame in Photoshop with the Overlay (I think) function in layers. Voila! Only one long wait instead of several.

10-01-2009, 02:42 AM   #5
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