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 Night time lapse shutter speed. 
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Joined: Mon Apr 13, 2015 12:17 pm
Posts: 3
Post Night time lapse shutter speed.
Hello.

I was wondering about the shutter speeds used on night time lapses. Until now I have always used 25 seconds exposures, but the problem is that with this exposure I can only make like 2 time lapses per night: 30 sec (25 exposure + 5 pause) x 300 frames = 2 hours 30 minutes. With this at best I can obtain two good time lapses per summer night.

But looking closely at my frames I can see clearly that the stars are a little bit turned into lines, and this got me wondering about the high exposure value that I'm using (25 seconds). What is the logic behind using such a high exposure value ? It is to gather light of course, I know, but how exactly does this process work ?

1. The light hits a pixel of the sensor.
2. Allowing more time for the light to hit that certain pixel, you collect more light on the sensor.

This is how I understand it. But what is the point of using exposures of 20, 25 seconds since the light moves to the next sensor pixel ? Because think about it, the Earth rotates and the star light that was hitting one certain pixel, now hits the next one, and so on. So in theory you no longer gather more light but you start the process all over with another sensor pixel until you get the stars looking like lines. So you gather more light through long exposures only as long as the light hits one certain pixel of the sensor. At the moment when the light moves to the next pixel, you no longer gather more light but you restart the process.

So wouldn't it be better to use lower exposure values ? Because in theory you can gather as much light as you can, and also save time.


Mon Apr 13, 2015 12:42 pm
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Joined: Wed Nov 11, 2009 10:45 am
Posts: 1685
Location: Merritt Island, Florida, Estates Unitas
Post Re: Night time lapse shutter speed.
You're absolutely right. Once your exposure is long enough that stars show up as little streaks, increasing exposure time will not make the streaks brighter or let you see fainter stars. Any landscape in frame will be brightened by the longer exposure though, as will larger sky features like dust clouds in the milky way.

If you do more timelapse videos each night, with each one being recorded over a shorter duration, you will get less movement of the stars onscreen. Its a creative decision. Seeing the stars moving ever so slowly is actually a cool effect because everyone else likes to show them moving fast.

Another possibility might be to get one or several cheap used cameras just to shoot some additional views. I started out with a couple of $200 ebay bodies. As I got better cameras, I kept the old ones for shooting additional views while my good cameras are doing the "real" shot. Timelapse is so automated, an ambitious shooter could have a dozen of them clicking away all night

Since I shoot a lot of timelapse, I also got several old M-42 screw mount lenses of different focal lengths. Whatever lenses I'm not using for the "real" shoot are available for the additional ones. I get cheap adapters without any electrical contacts to attach them to my camera. I set the camera to shoot without lens attached so it will fire the shutter without electrical connection to the lens. The screw mount lenses had the advantage of being manual all the way. You had to manually open the aperture to focus, then manually close it down to shoot. It does not automatically jump from full open to the set aperture for every exposure, so there is no aperture flicker. It behaves just like a cine lens.


Mon Apr 13, 2015 5:26 pm
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Joined: Mon Apr 13, 2015 12:17 pm
Posts: 3
Post Re: Night time lapse shutter speed.
I understand, so in theory if I want to see the Milky Way clouds I should leave the exposure the way it is. I've made some tests tonight from my window because the sky got clearer and from what I've seen with 10 seconds the stars no longer appear as lines.

I'm shooting with Canon 600D + Tamron 17-50 f/2.8. For the future I have in plan to buy two more 600D cameras, a Samyang 16mm T2.2 (f/1.8) and a Samyang 10mm T3.1 (f/2.8).

Do you think that with my current setup and a 10 seconds exposure I could see the Milky Way (this I can't test from my window because of light pollution and requires a mountain trip) ?


Tue Apr 14, 2015 8:25 pm
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Joined: Tue Jan 11, 2011 8:12 pm
Posts: 121
Post Re: Night time lapse shutter speed.
Additional image brightness can be achieved through 'image stacking'. Multiple images are taken, micro-aligned to match the star positions, and then added together. I've never done it, but I think that's one of the keys to astrophotography if you don't have a tracking mount.


Wed Apr 15, 2015 7:44 am
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Joined: Wed Apr 03, 2013 7:20 pm
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Location: Brisbane, Australia.
Post Re: Night time lapse shutter speed.
How about you take that mountain trip and go see for yourself? Then you'd know.

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Wed Apr 15, 2015 2:44 pm
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Joined: Mon Apr 13, 2015 12:17 pm
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Post Re: Night time lapse shutter speed.
Right now it can't be done, that is precisely why I was asking in the first place, bad weather and very cold, especially at night, clouds and haze in the air. Yesterday I could see a few stars briefly and I tested the shutter exposure time, but I could not test to see the Milky Way because of light pollution.


Wed Apr 15, 2015 3:02 pm
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Joined: Wed Oct 16, 2013 4:07 am
Posts: 306
Post Re: Night time lapse shutter speed.
here's a website i like it. may make things clearer for you perhaps http://www.lonelyspeck.com/lenses-for-m ... otography/
hope it helps.


Wed Apr 29, 2015 2:12 am
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Joined: Mon Feb 20, 2012 4:14 am
Posts: 4
Post Re: Night time lapse shutter speed.
Increasing aperture or ISO will increase star size for a given exposure length and stacking images is done to increase signal to noise ratio, meaning noise is canceled out as most is random in nature. This is why stacking is done, it will not increase a star's brightness, that is controlled by exposure. (although increasing S/N ratio will make them stand out better due to increased local contrast)

JA


Wed May 06, 2015 1:14 pm
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Joined: Wed Jan 20, 2010 4:50 pm
Posts: 561
Post Re: Night time lapse shutter speed.
Hi Critical Point. Going back to your original interesting question - the idea that because a star is moving, it only effects one pixel at a time, and each pixel only receives a limited amount of light.

I think the answer is that any one star of any size is going to effect more than one pixel. Some of the smaller cloud like star masses may only represent one pixel, but major stars should cover a bit more. (Due to lens dispersion and effects on the CCD itself.) In addition, each "pixel" is actually made up by averaging several pixels with different colours.

Hence leaving the shutter open longer will make even the major stars brighter up to a point. - ie. within a small range of movement they will build up the exposure, before moving far enough to completely effect a new group of pixels. So there will be a certain "peak level" for individual stars. But what will change is the overall background level. The general sky and milky way exposure will build up and begin to show more depth... so the scene will look brighter all the same...

Ultimately it depends on your judgement what is the best exposure. This will also depend on how high you feel you can push the ISO levels before it becomes too noisy...


Wed May 06, 2015 1:55 pm
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Joined: Mon Feb 20, 2012 4:14 am
Posts: 4
Post Re: Night time lapse shutter speed.
Right, a star can definitely see a whole lot more more than one pixel, the exact amount dependent on the sample ratio of the sensor pixel size and lens combo (in arc seconds per pixel)


Thu May 07, 2015 7:49 am
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