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Exposure time and "busy" time.

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Exposure time and "busy" time.
« on: 06 / January / 2009, 05:47:37 »
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I've been experimenting with long exposures (both overridden CHDK style long (64 second) and original firmware long (15 second)) and noticed something I thought I'd ask about. For every exposure, the actual time taken seems to be twice the shutter speed - 1 second exposure takes 2 seconds, 5 second exposure takes 10 seconds. I was trying to do some "light writing" with a torch on a dark scene but couldnt for the life of me determine when the shutter was actually open during the total time. I assumed it was the first half, with the second half "processing" in some way, because there is an initial shutter-like click and then one at the half way point, but then there is also one at the end.

Can anyone expand on the process involved here - I dont know when I should be wafting my torch about :)

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Offline Hacki

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Re: Exposure time and "busy" time.
« Reply #1 on: 06 / January / 2009, 05:57:02 »
Its a darkframe substraction. You can check what happens without the darkframe substraction for yourself, go to "Raw Parameters" and set the "Dark Frame Substraction" option to "Off" - the camera then wont take the darkframe and present the photo directly to you.

Little explanation: The longer the exposure time, the warmer the sensor and its surrounding parts get - they introduce artifacts known as "amp glow". With very long exposures, this amp glow can become that strong that 1/4 of your image is bright purple, even when you close the lens with your hand. The darkframe takes a picture of the same exposure with a closed shutter, and substracts it from your real photo - that way amp glow and some static hot pixels should get removed.


Re: Exposure time and "busy" time.
« Reply #2 on: 06 / January / 2009, 06:05:17 »
Ahh ok, that explains it perfectly. So in theory my torch wafting after the initial period should make no difference as the mechanical shutter should be shut even though the camera is doing a "second exposure" in effect? I presume the shutter is shut or you'd end up with some nice black images when the subtraction was done :)

Re: Exposure time and "busy" time.
« Reply #3 on: 06 / January / 2009, 06:07:52 »
Hmm my build doesnt have dark frame subtraction off option I dont think, unless its been renamed noise reduction. I'll have a play and see!


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Offline Hacki

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Re: Exposure time and "busy" time.
« Reply #4 on: 06 / January / 2009, 06:21:06 »
Then you have a very old build! It was renamed from "Noise reduction" to "Darkframe substraction" .. quite a while ago. Cant find when exactly.

Get a new one from CHDK Download - build 0.9.0, revision #666


 

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