Lightning with Intervelometer vs MD - Creative Uses of CHDK - CHDK Forum
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Lightning with Intervelometer vs MD

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Offline CanonBlue

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Lightning with Intervelometer vs MD
« on: 21 / April / 2012, 11:26:38 »
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I have taken pictures of lightning before using the chdk motion detect script. The other day I decided to try the intervelometer instead. I set it up to just keep taking photos, and I set the exposure to be as long as the built in canon settings would allow - set to 15. The problem is after a 15 second exposure, the camera seems to take about 15 seconds to process it! So I figured there was still about a 50% chance of any particular lightning bolt landing in a shot. I left it in the window for a couple hours while a storm passed over, and I only got one shot, but I really like it. I think I will try this method again. Another thing is maybe I should try using the chdk to make the exposure times even longer, but I was worried about the photos getting too bright and washed out. As you can see, this one is already decently bright. (I did no post processing on it at all.)

IMG_2682

Time will tell whether its better or worse than motion detect. Anyone have thoughts on that? I noticed the MD was activating slower on my newer camera, and I was worried it wouldn't be fast enough to catch lightning. Of course, if there are lots of strikes one after another, the MD might still work very well. I think I'll experiment with both as the summer storms come.
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Offline waterwingz

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Re: Lightning with Intervelometer vs MD
« Reply #1 on: 21 / April / 2012, 11:55:24 »
Are you saving RAW/DNG images ?  That will really increase the time to save an image. 

Also,  you can try turning off Dark Frame Subtraction - its in the RAW menu but turns it off for the JPG too.  You will have to see if it hurts the image but should shorten your save times.
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Offline CanonBlue

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Re: Lightning with Intervelometer vs MD
« Reply #2 on: 21 / April / 2012, 12:32:00 »
No, I'm not saving RAW or DNG files. The camera takes a long time to process long exposures even when I'm not running the chdk.
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Offline waterwingz

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Re: Lightning with Intervelometer vs MD
« Reply #3 on: 21 / April / 2012, 14:04:50 »
Quote from: CanonBlue on 21 / April / 2012, 12:32:00
No, I'm not saving RAW or DNG files. The camera takes a long time to process long exposures even when I'm not running the chdk.
You will find than is saves faster that it does when you don't use CHDK if you go into the RAW menu (even though you are not using RAW) and turn off Dark Frame Subtraction.   Use the search function on this forum and look for "Dark Frame Subtraction" if you want more detail.  Or just try it and see.
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Offline outslider

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Re: Lightning with Intervelometer vs MD
« Reply #4 on: 21 / April / 2012, 14:20:06 »
The problem is that when camera shoots with long exposure it actually takes two 'photos':

- first one is with opened shutter - camera takes light from the scene to produce image
- the second is taken with closed shutter - camera takes so called dark frame.

The aim of such behavior is to remove some sort of noise and artifacts that could appear in so long exposure. Camera takes dark frame of the same time that it takes photo. So the 15 sec when camera says 'busy' is not really a processing but dark frame shoot.

Read more:
http://en.wikipedia.org/wiki/Dark-frame_subtraction

When you use CHDK you can turn this feature off - which waterwingz has suggested in his post. You can do this in RAW menu.

EDIT - I was beated:D
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Offline CanonBlue

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Re: Lightning with Intervelometer vs MD
« Reply #5 on: 23 / April / 2012, 09:39:52 »
Interesting, I never knew what it was doing when it said busy. I will definitely try that next time there's a storm. In fact I will just try it tonight when its dark out, to see if it adds obvious glare or problems to the photo. It seems like there should be a more efficient way to do this - if I am taking what's basically the same long exposure photo over and over again, couldn't we set it up to do the dark frame subtraction just once, or once every 10 shots, instead of every single shot?
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Offline reyalp

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Re: Lightning with Intervelometer vs MD
« Reply #6 on: 23 / April / 2012, 12:49:17 »
Quote from: CanonBlue on 23 / April / 2012, 09:39:52
It seems like there should be a more efficient way to do this - if I am taking what's basically the same long exposure photo over and over again, couldn't we set it up to do the dark frame subtraction just once, or once every 10 shots, instead of every single shot?
With raw, this is common practice. Most raw processing programs let you specify a darkframe to use in your workflow. Note that for best results, the dark frame should be taken at exactly the same exposure settings and sensor temperature as the original shot.

With jpeg, the results aren't likely to be as good. The artifacts you are trying to subtract often affects individual red, green or blue pixels, but in the jpeg they've already been combined. Worse, jpeg compression will spread them around in unpredictable ways. It might be better than nothing.

We could theoretically load pre-saved darkframes from disk and do the subtraction before the jpeg was saved, but this would be slower than just saving a raw.
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