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PostPosted: Wed May 20, 2009 9:04 am 
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I found myself trying to do this a couple of weeks ago. Here are my results:
F/4.5, 30 secs, ISO 800 18 mm
Image
F/3.5 30 secs ISO 200 18 mm
Image

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PostPosted: Wed May 20, 2009 4:22 pm 
love the colours is that natural ?

how much photoshop?


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PostPosted: Wed May 20, 2009 5:06 pm 
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Just some whitebalance shifting. The light pollution of my town gave me the nice red glow :)

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PostPosted: Tue Nov 03, 2009 10:48 pm 
I've been doing a little reading and read through this entire thread. My question is, what's the difference in ISO setting? What's the difference between ISO 100 Vs. 1600? (I'm using a Canon Rebel XS, so that's as high as mine goes). Also, What about the function Long Exposure Noise Reduction? I noticed that the photos in this thread were mostly ISO 800 and higher, but how come on this thread http://www.cameralabs.com/forum/viewtopic.php?t=17875 ISO is around 100?

No matter how much I read, I can never seem to understand 100% how ISO works.


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PostPosted: Wed Nov 04, 2009 1:17 am 
As far as I'm aware, ISO setting is simply the gain used when converting the information from the CCD/CMOS chip to a digital RAW file. The more the pixels are amplified (more gain), the brighter the image will be. Of course the noise will be amplified as well.

For the moon shots you saw in the thread you linked ISO 100 was sufficient as the moon is a much brighter target than you might think, especially trough a telescope. For nebula an ISO setting of 800 to 1200 seems to be about optimal.

The one thing I myself am not entirely sure about is what the ISO speeds themselves mean. They appear to be a relic from the analog age, this means the ISO speed for one chip is the same for another chip. But to do this you would need to use different actual gain values to achieve the same ISO speed. A 15 megapixel full frame sensor will have relatively large pixels, making it quite sensitive already meaning no need for a very high gain to achieve the same ISO speed. Somebody else will have to confirm this last paragraph though...

"Long Exposure Noise Reduction" is most likely a post-processing algorithm inside a digital camera that can be turned on or off. Like all post-processing this can be done after taking the picture on your computer assuming you have the RAW file.


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PostPosted: Wed Nov 04, 2009 4:13 am 
Zoeff wrote:
As far as I'm aware, ISO setting is simply the gain used when converting the information from the CCD/CMOS chip to a digital RAW file. The more the pixels are amplified (more gain), the brighter the image will be. Of course the noise will be amplified as well.

For the moon shots you saw in the thread you linked ISO 100 was sufficient as the moon is a much brighter target than you might think, especially trough a telescope. For nebula an ISO setting of 800 to 1200 seems to be about optimal.

The one thing I myself am not entirely sure about is what the ISO speeds themselves mean. They appear to be a relic from the analog age, this means the ISO speed for one chip is the same for another chip. But to do this you would need to use different actual gain values to achieve the same ISO speed. A 15 megapixel full frame sensor will have relatively large pixels, making it quite sensitive already meaning no need for a very high gain to achieve the same ISO speed. Somebody else will have to confirm this last paragraph though...

"Long Exposure Noise Reduction" is most likely a post-processing algorithm inside a digital camera that can be turned on or off. Like all post-processing this can be done after taking the picture on your computer assuming you have the RAW file.

So in a nutshell, people use lower ISO because the moon is a bright enough source of light? :D


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PostPosted: Wed Nov 04, 2009 7:15 am 
Indeed, no need to amplify all that noise when there's a good enough signal (enough light) that you can work with. :)


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PostPosted: Wed Nov 04, 2009 11:31 am 
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Hi folks,

It's been a while since I dabbled in astrophotography but my own inclination was to use ISO 400 when shooting a sequence of long exposures intended for stacking later on. In a DSLR all the ISO setting does is to change the gain of the readout amplifier(s) so setting the ISO too low risks losing the signal while setting the ISO too high risks clipping the brightest objects while not improving the signal to noise ratio of the faint stuff. But if one is shooting an area without any comparatively bright objects (compared to the main target object) then maybe experimenting with a higher ISO would be worthwhile. :?

As for long exposure noise reduction, turn it off. You can take "darks" at any time that's convenient provided the camera temperature is about the same whereas taking 8 minutes to do every 4 minute exposure in a sequence risks losing shots unless you are blessed with reliably clear skies. :roll: I also prefer the option of doing dark frame subtraction during post-processing as one has better control and potentially better tools.

Bob.

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PostPosted: Thu Nov 05, 2009 5:18 pm 
I decided to experiment last night. I was doing it in an area of much light so the light pollution is obvious. I just wanted to see what I might have been able to produce. I was quite shocked too. Here's what I was able to get.

Image

Its very interesting because the camera detected stars that the naked eye couldn't even see. I also messed around with the ISO. The higher ISO, the more stars appeared.

Interesting stuff. And I have to say, it's quite addicting!


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PostPosted: Thu Nov 05, 2009 6:26 pm 
AP is indeed quite awesome. You can't tell your eyes what to do, but you do have complete control over CCD chips.

If you look closely at each individual star, you can even see the rotation of the earth. :D


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PostPosted: Fri Nov 13, 2009 9:13 am 
Quick question...what kind of shutter speed are you guys using?
I find that anything over 4s results in noticeable star trails, in contrast to the 20-30s that you guys have been using. This means that even with a f1.8, ISO 3200 im having trouble getting a full starfield.

I've tried stacking with Iris, but it doesn't do all that much, just reduce the noise a tad (im probably doing things wrong though...), and actually reduce the number of stars..


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PostPosted: Fri Nov 13, 2009 9:30 am 
How bad are the star trails? All images of this type show some degree of trailing. Even the resized shot posted above has slightly oval shaped stars.


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PostPosted: Fri Nov 13, 2009 10:08 am 
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.
There's a formula and a handy table here. Note that the lens focal lengths quoted in that table are the 35mm film equivalents so if you were using a 25mm lens (actual focal length) on a camera with a crop factor of 2x you should enter the table at 50mm.

The arithmeticians can take the formula a bit further, of course, and calculate the exposure time needed to trail a star at a particular declination by an arbitrary fraction of the inter-pixel distance for their particular camera... ;)

Bob.

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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PostPosted: Fri Nov 13, 2009 12:22 pm 
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Rather than calculate it, I'd just try it and see what works in practice. The wider the angle you go, the more time you can take before trails start moving. Do note the stars move at different rates across the sky, and in different directions too if you're wide enough e.g. as you move away from the equator the stars will curve in different directions. With stacking, I think this gives an effect where parts of the image are sharp but other regions get slight trailing. This is less of a problem if you image a relatively smaller area.

Roughly speaking, at 10mm on crop sensor 30 secs is ok, but at more typical wide angles of 16-18mm I prefer to drop down to 15-20 secs or so. I find it incredibly difficult to get pin sharp focus with ultra-wide angles so the misfocus helps hides trails a bit.

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PostPosted: Sat Nov 14, 2009 5:14 am 
Oops. Its my tripod's fault. (Also mine in a way..). I normally shoot out my window, and I don't have enough room to fully extend the legs. One of the clamps is loose, and it shifts. Tightened it up, less 'star trails' now. :oops: :oops:

I get about 8s exposures now, a big improvement

Quick picture of what it was like before:

Image
f/1.8, 5s, ISO3200 (NR enabled)


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