Save2file during shutter is open - Feature Requests - CHDK Forum supplierdeeply

Save2file during shutter is open

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Save2file during shutter is open
« on: 03 / June / 2008, 12:01:10 »
Is it possible, to save a picture while performing a long time shot? I'd like to have many pictures from the same starting time but different durations. Best regards, Manuel.


Offline BB

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Re: Save2file during shutter is open
« Reply #1 on: 03 / June / 2008, 13:20:53 »
Yes and no....

First, there are some special issues that you need to understand...

1. When you commit the camera to take the picture (say 64 seconds)--it will do nothing until the picture taking process is completed and it has saved the file to the card.

2. When taking long exposures (2 seconds or longer), there are actually two photos being taken. The first, for 64 seconds, and a second, also for 64 seconds, but with the shutter closed. This type of Noise Reduction is called "Dark Frame Subtraction". What the camera does is take the first exposure (your "photo") and subtracts the 2nd (dark frame) from your "photo". This helps remove things like "hot pixels" and Amp Noise (amplifier noise--typically a purple haze on a portion of the frame--gets worse with higher temperature and longer exposures). The result is then converted to JPEG and saved to disk.

So, that is the process that cannot be interrupted once it gets started.

Now, with CHDK you have some other options. First, you can turn on RAW and avoid the JPEG/NR processing. Second, you can turn off "NR" in the CHDK menu (really Dark Frame Subtraction) and avoid the 2nd 64 second delay... Some folks may take 5 pictures then 1 dark frame--allows you, for example, to take sky shots for a longer period of time, but still have dark frame subtraction available to keep the internal camera noise down.

There is another thread here (or two) where people are mounting a CHDK camera to a telescope with motorized tracking--and using Frame Stacking (I think that is what it is called) where you can stack 5x 64 second exposures for the equivalent of a 150 second exposure. And you only had 1x 64 seconds wasted in dark frame photography.

So, you can use a series of photographs (at any shutter speed) and take any number/time frame of them and use the frame stacking software to make the equivalent of different "time or bulb" exposures (different start/stop times) from one string of (basically) time-lapse type photographs.

Does this make sense and answer your question?


PS: Here is Frame Stacking using CHDK bracketing on focus so you can make an unlimited depth of field photograph:

RAW average vs Helicon Focus?

Long exposure with tracking telescope:

Night sky 60 second exposures

Re: Save2file during shutter is open
« Reply #2 on: 03 / June / 2008, 13:33:08 »
Thank you very much! I will try the solutions soon.


Offline fe50

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Re: Save2file during shutter is open
« Reply #3 on: 03 / June / 2008, 16:12:39 »
Hi Manuel,
nice to meet you again here in the international forum.

Re: Save2file during shutter is open
« Reply #4 on: 04 / June / 2008, 03:39:55 »
Yeah, thanks for your advisory!
I asked in the German Forum before. I want to take photos of fire juggling, before i used CHDK i got the best results with 2sec long time shots, but the outcome always depends on luck. I will tell you about the results...


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