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08-10-2016, 03:00 PM   #1
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K1 optimal ISO for Astro

When we increase ISO, noise increases but exposure time decreases. When doing astro photos, we have to trade off iso vs shutter speed for the same exposure amount, we are the choice to use lower ISO and longer exposure , but with long exposures the image noise from dark currents increases. At which level the high iso noise reach the level of dark current? So what is the best ISO setting for night photos?

08-10-2016, 04:21 PM   #2
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I don't own a K-1 but from what I hear the high ISO/low noise ratio is quite good. That is one reason why I will get a K-1 but not for stars.

I will basically say though that it's all going to depend on what kind of look you are going for...but I would imagine with something like a 31mm just figure out the shutter speed that is fast enough to avoid trails and go from there.

There is some kind of formula people put up from time to time that is focal length based but I can't remember what it is.

I would venture to say that the K-1 would give you a whole lot better options for shooting stars than anything we've previously had.

Between the better ISO, lower noise, and better dynamic range... I would think it's worth some experimentation.
08-10-2016, 05:19 PM   #3
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You might want test to with something like 400 ISO for possibly 1-2 minutes and fluctuate if needed.
08-10-2016, 05:50 PM   #4
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QuoteOriginally posted by biz-engineer Quote
When we increase ISO, noise increases but exposure time decreases. When doing astro photos, we have to trade off iso vs shutter speed for the same exposure amount, we are the choice to use lower ISO and longer exposure , but with long exposures the image noise from dark currents increases. At which level the high iso noise reach the level of dark current? So what is the best ISO setting for night photos?

If I understand what you are asking here, you needn't worry about dark current if you are using dark frame subtraction (slow shutter noise reduction).

That being said, I often play in the 400-1600 ISO range with my K5 IIs - similar pixel density, likely similar results. A recent one looking over the Ottawa River after moonset. As noted by others I find the trackign ability is hindered by vertical orientation.

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08-10-2016, 05:52 PM   #5
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I guess the right people will come along with enough expertise soon, until then:

I have used the K-1 for Milky Way images and have used ISO 1600 at f2.8 and 30secs to get a reasonable exposure. This has generated some star trails on the edge of the frame at 15mm and 21mm, so I've dropped that time to 10-15sec and ISO 3200 and can achieve similar results like this:



I was out two nights ago chasing the Milky Way again but trialing the astrotracer for which I dropped to ISO 400 & 800 for approx 4:30secs / 3omm and f2.8 to get an idea of what seems to work best with that feature. TBA on that one.

A good website to review settings for Milky Way photography is Royce Bair's: Into The Night Photography his Flickr page is here: https://www.flickr.com/photos/ironrodart/ You can see from his EXIF that he's at 25secs and ISO 6400 using the 5D Mk III and the Tamron 15-30. He has some very good information about camera settings on some of his Flickr images too.

Of course if you're talking different astro like deep space stuff this technique, as demonstrated on the Milky Way is the next approach I intend to try to address noise: Video: Noise Reduction with Image Stacking – Lonely Speck

Something I'm finding too is that a lot of the noise I'm capturing can be controlled by sepcific techniques in PP, so if you're looking at trying this yourself I'd recommend pushing the ISO to 3200-6400 for a 10-15sec exposure at f2.8. In PP I use LR6, turn off the default sharpening then use some NR to taste. I return to the basic panel in LR6, or you could use the curves tool and work the highlight/shadow and blacks sliders to further reduce the noise. Note also that I don't use the dark frame subtraction. With the shorter exposures it's easier to try both and see which one you prefer.

Hopefully some of this info will be of assistance.

Tas

Last edited by Tas; 08-10-2016 at 05:58 PM.
08-10-2016, 07:12 PM   #6
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You can find a good discussion on the subject here. Just remember that the sensor doesn't know the iso setting you chose when taking the picture. Iso setting is just amplification gain applied after the shutter is closed. Toying with Raw digger I found that my K-30 clips the dark part of the histogram below iso 400 in order to protect the highlights I suppose. Iso 800 shows a drop in EV range which gets a lot worst as you increase iso, this saturates the stars. So I generally use iso 400 or 800 and boost the signal in Camera Raw, sometime three steps, bringing highlights slider down. If you boost an iso 400 image 3 steps you'll get as much noise as an iso 3200 image but with a much wider EV range. The funny thing is if you take an iso 6400 image and decrease exposure 1 step, you'll get slightly less noise than the original iso 3200 or the boosted iso 400 images!
08-10-2016, 08:48 PM   #7
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QuoteOriginally posted by mattt Quote
That being said, I often play in the 400-1600 ISO range with my K5 IIs - similar pixel density, likely similar results. A recent one looking over the Ottawa River after moonset. As noted by others I find the trackign ability is hindered by vertical orientation.
What do you mean by this -- "vertical orientation"? Just shooting th image in portrait mode? Or the combination of both the sky and terrestrial objects in the frame?
08-10-2016, 11:11 PM   #8
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Lot of information guys, thank you so much. I have to read it though + the links and digest it :-)

08-11-2016, 04:27 AM   #9
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QuoteOriginally posted by leekil Quote
shooting th image in portrait mode?
yes - this - I'm shooting a bit wide and long here - first go at meteors you see.
08-11-2016, 06:11 AM   #10
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Photographic Dynamic Range versus ISO Setting

The dynamic range (latitude between overexposure and noise floor) are quite linear with ISO. I think that means the lowest ISO are an advantage unless you simultaneously choose to lengthen the exposure time or store your images in jpeg in camera. Low ISO means you get the same noise when pulling the "shadows" equally to a higher ISO, but preserve the highlight colors (star colors) much better.

I would have tried ISO 100 as a basis, and then try to find the sweet spot of how many shots you should divide your total exposure time to. That also depends on the use of astrotracing or dedicated tracker and temperature. High temperatures give shorter optimal exposure times. Tracing is a must for longer exposures, ant the longer the focal length is the more important it is.

Take X shots of Y seconds and stack them. Try other combinations of X and Y that gives the same total exposure time to find the optimal settings for your temperatures, focal lengths and tracking.
08-11-2016, 04:34 PM   #11
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The issues of calibrating AstroTracer and juggling ISO vs length of exposure aside, another issue you'll face is the white dot problem that affects the K1 (at least in mine and several other owners of the K1 that I am personally aware of) that was also a problem in the Nikon D810 (same basic sensor as the K1). A long exposure of a minute or more at higher ISO and the dots appear against dark backgrounds. The long shutter NR is a band aid at best and covers up the problem and also takes the camera out of use while NR is applied. A real bummer when you're trying to do 1-4 minute exposures during a meteor shower or trying to create star trails.
08-11-2016, 05:11 PM   #12
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This white dot thingy I can't see on any of my K-1 3-4 minute images. Where do I find info on the subject?
08-11-2016, 11:33 PM - 1 Like   #13
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QuoteOriginally posted by Tjompen1968 Quote
This white dot thingy I can't see on any of my K-1 3-4 minute images. Where do I find info on the subject?
Follow the link in the first post here:https://www.pentaxforums.com/forums/190-pentax-k-1/327326-k-1-white-dot-issue.html

Or do a search for "D810 white dot" or "D810 white spots" or even "D810 hot pixels".

Here is one link https://photographylife.com/nikon-confirms-the-d810-thermal-issue-and-offers-a-solution
08-12-2016, 12:25 PM   #14
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The discouraging thing about this issue is that Nikon, with factory inhouse service and staff, was equipped to handle the issue. Pentax is not. They farm out their repairs and when you call Ricoh-Pentax imaging you typically get someone not familiar with the problem or the camera to any great extent. That person tries to transfer your call to Colorado and invariably that one individual in Colorado is out to lunch, just returned from vacation and will call you later, etc. I've heard a number of excuses at this point but I've never received a call back. Very discouraging.

It seems obvious that Pentax selected the same SONY sensor that created the problem for Nikon. I have no confidence that Pentax will solve the issue anytime soon other than telling everyone to use long shutter noise reduction, a band aid.
08-12-2016, 12:43 PM   #15
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QuoteOriginally posted by CDW Quote
The discouraging thing about this issue is that Nikon, with factory inhouse service and staff, was equipped to handle the issue. Pentax is not.
I am semiconductor engineer. Nikon can't fix sensors, neither Pentax. So, you can be assured that if you have a different perception, it's only a perception. If some pixels have more dark current than the statistical average, no one can fix it with a cheap method, and what's done by the camera software with the sensor dead pixels are scanned if the mask it out from the image but the defect is still there.
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