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Author Topic: noise (high ISO vs. long exposure)  (Read 24608 times)
philbond87
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« on: December 08, 2006, 12:07:42 PM »
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Is noise the same across the board?
What I meant to ask is, would the noise levels resulting from using a lower ISO with a longer exposure be the same as that same exposure using a higher ISO and correspondingly shorter exposure?

"Test for yourself" would be a reasonable response to this question, I know, and I intend to.  However, I would like to know the theory behind it, if anyone here happens to know.

Intuitively I would suspect that the noise would be different, however logically I might assume otherwise.

Thanks.
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dobson
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« Reply #1 on: December 08, 2006, 01:07:25 PM »
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This is an interesting question. A comparison of ISO noise and thermal noise. I have a feeling that for any situation you will have complicated answers. The answer will vary a lot based on whether you use dark frame subtraction or not, ambient temperature, temperature of the sensor, etc. If I find myself bored some night I will do some testing and share some detailed results.

Phillip
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bjanes
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« Reply #2 on: December 08, 2006, 02:18:40 PM »
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Is noise the same across the board?
What I meant to ask is, would the noise levels resulting from using a lower ISO with a longer exposure be the same as that same exposure using a higher ISO and correspondingly shorter exposure?

Intuitively I would suspect that the noise would be different, however logically I might assume otherwise.

Thanks.
[{POST_SNAPBACK}][/a]

Under most conditions, the total noise is related to the exposure, which determines the number of photons collected. A long exposure at a low ISO will collect more photons and will have greater signal to noise, even though the absolute noise is higher. Sensors with large pixels also collect more photons and will have less noise if other factors are the same.

A digital camera changes ISO by amplifying the signal. Fewer photons are collected and noise is higher. Some noise is contributed by the amplification and reading of the charge of each pixel. This is called read noise, which can be reduced at higher ISO. This type of noise involves mainly the darker portions of the image. If light is limited, there is an optimum exposure.

Thermal or dark current noise varies with temperature and becomes a factor only when exposure is many seconds or longer. It is not a significant factor in most situations.

For a good analysis, see Roger Clark's web site:

[a href=\"http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary/]http://www.clarkvision.com/imagedetail/dig...rmance.summary/[/url]
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philbond87
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« Reply #3 on: December 08, 2006, 02:24:03 PM »
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Thank you!
That is precisely the explanation I was looking for.

Regards,
Phil
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Chris_T
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« Reply #4 on: December 11, 2006, 09:41:58 AM »
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Under most conditions, the total noise is related to the exposure, which determines the number of photons collected. A long exposure at a low ISO will collect more photons and will have greater signal to noise, even though the absolute noise is higher. Sensors with large pixels also collect more photons and will have less noise if other factors are the same.

A digital camera changes ISO by amplifying the signal. Fewer photons are collected and noise is higher. Some noise is contributed by the amplification and reading of the charge of each pixel. This is called read noise, which can be reduced at higher ISO. This type of noise involves mainly the darker portions of the image. If light is limited, there is an optimum exposure.

Thermal or dark current noise varies with temperature and becomes a factor only when exposure is many seconds or longer. It is not a significant factor in most situations.

For a good analysis, see Roger Clark's web site:

http://www.clarkvision.com/imagedetail/dig...rmance.summary/
[a href=\"index.php?act=findpost&pid=89453\"][{POST_SNAPBACK}][/a]

In Stephen Johnson's book, he talked about sensor noise from long exposures. But he was not specific what he meant by "long".

He did mention some cameras have "dark current measurement/subtraction functions" to deal with noise from long exposures. The idea seems to involve creating a "dark frame, an image of the sensor with no light striking it" (at long exposure?) and then subtracting it from the image to remove the noise.

Is there a common name this feature is referred to by the manufacturers? Is it commonly found on most models?
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gochugogi
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« Reply #5 on: December 11, 2006, 11:27:32 AM »
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It depends on how long... I've shot a lot of night scenes at ISO100 (10D & 5D) and anywhere from a few seconds to 30 seconds and the scenes are amazingly low noise--nearly "grainless." However, if I crank the ISO, even a few stops, the apparent noise increases greatly.

I haven't shot any startrails but I'm sure as exposure times increases noise will increase. I'm just not sure at which point a high ISO is an advantage noise-wise over a longer exposure. I do know it's somewhere beyond 30 seconds for the 10D and 5D.

This image was shot at ISO100, F11 and 15 seconds (10D). It is virtually noise-free even as 11 x 14 print. I also shot shorter exposures of the same scene at ISO 400 and 800. They were terribly gritty.

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John Sheehy
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« Reply #6 on: December 11, 2006, 04:52:27 PM »
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I don't see my reply in this thread - I thought I posted one; well anyway:

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Is noise the same across the board?
What I meant to ask is, would the noise levels resulting from using a lower ISO with a longer exposure be the same as that same exposure using a higher ISO and correspondingly shorter exposure?
[a href=\"index.php?act=findpost&pid=89425\"][{POST_SNAPBACK}][/a]

The important things to realize are:

1) Dark current noise (related to exposure time and temperature) exists relative to signal in the analog sensor wells, and the ratio has absolutely nothing at all to do with the ISO setting.

2) ISO-related noise exists relative to RAW exposure (the dynamic range of the RAW data at that ISO).

3) Depending on the camera, there may be more noise at higher ISOs relative to absolute signal, the same, or less noise.

#3 is very important to know for your camera, but I don't know if this data is actually listed anywhere for specific cameras.  I know that for recent Canon models, the lowest ISOs have the highest noise, relative to absolute signal.  This means that you can consider absolute exposure first, and use ISO accordingly, to make sure that the ISO chosen lets your signal give a good exposure for that ISO.  If your camera does not improve in noise efficiency at higher ISOs, then it may not be worth using the higher ISO, since it will not give you any less noise.

As far as the trade-off between time and signal-to-dark_current_noise is concerned, in most of my experience, dark current noise does not quite double in double the time, so the longer exposure is generally better; IOW, with the same f-stop, you are better off with 8x the exposure time at ISO 200 than 1x at ISO 1600.  (ISO 100 is not always a good ISO for highlights, depending on the camera).

A more objective experiment, however, would be in order.  I would expect to see any advantage for the shorter exposure (all other things being equal) to only happen at very long exposures (multiple minutes), because there is *always* more noise with a higher ISO or under-exposed/pushed lower ISO that has nothing to do with dark current.  In my recent Canon 20D test, ISO 100 at 30 seconds was better than ISO 1600 at 2 seconds, but clearly part of the benefit was the lower non-dark_current noise at ISO 100.  The hottest pixels were slightly hotter in the 1600 image.
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Idefix
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« Reply #7 on: December 14, 2006, 01:37:46 AM »
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Perhaps some useful links for high iso speed photos:
http://highiso.net

But there's just test samples I think.
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Jonathan Wienke
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« Reply #8 on: December 14, 2006, 02:21:56 AM »
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Here are some universal principles of exposure and ISO with digital cameras.

Shoot at the lowest possible ISO that allows an acceptably short shutter speed. Shooting a long exposure at low ISO gives a cleaner result than a shorter exposure at correspondingly higher ISO. If you are shooting static subjects from a tripod, use the lowest native ISO you can.

Shooting a high ISO and exposing properly is better than shooting at a lower ISO, underexposing, and push processing.

Avoid ISO settings that are not native to the camera sensor, i.e. that are achieved by processing tricks in-camera and not by the sensor itself. An artificially low ISO setting, like 50/L on the 1Ds, sacrifices dynamic range. You're no better off than shooting at ISO 100 with regard to capturing highlights. An artificially high ISO, like 3200/H on the 1D-MkII is the same as underexposing and using a positive exposure compensation setting in the RAW converter.
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jule
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« Reply #9 on: December 17, 2006, 03:06:00 PM »
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Avoid ISO settings that are not native to the camera sensor, i.e. that are achieved by processing tricks in-camera and not by the sensor itself.
Jonathan, How do you know which ISO settings are native to the camera?

Julie
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Jonathan Wienke
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« Reply #10 on: December 18, 2006, 02:45:11 AM »
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As a rule of thumb, it it is an extended ISO setting that has to be enabled in the camera menu (ISO expansion, etc) before it can be used, then it's probably fake.
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John Sheehy
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« Reply #11 on: December 18, 2006, 08:08:26 AM »
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Jonathan, How do you know which ISO settings are native to the camera?
[a href=\"index.php?act=findpost&pid=91026\"][{POST_SNAPBACK}][/a]

Well, you won't see this information in the manual, usually.  The RAW data has to be analyzed to tell how it was derived.  In some cases, there is little or no negative effect from using a mathematically-derived ISO setting, but in others, you can do better by using the more real ones.
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Ray
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« Reply #12 on: December 18, 2006, 08:29:32 AM »
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Well, you won't see this information in the manual, usually.  The RAW data has to be analyzed to tell how it was derived.  In some cases, there is little or no negative effect from using a mathematically-derived ISO setting, but in others, you can do better by using the more real ones.
[a href=\"index.php?act=findpost&pid=91154\"][{POST_SNAPBACK}][/a]

From what I understand from John's previous posts on this subject, ISO settings have traditionally moved in double numbers, ISO 100, 200, 400, 800 etc.

In the latest Canon models, we have fractions of this. ISO 250, 320, 640 etc.

These are the non-native ISOs that serve no real, purpose.
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Chris_T
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« Reply #13 on: December 18, 2006, 11:44:55 AM »
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Here are some universal principles of exposure and ISO with digital cameras.

Shoot at the lowest possible ISO that allows an acceptably short shutter speed. Shooting a long exposure at low ISO gives a cleaner result than a shorter exposure at correspondingly higher ISO. If you are shooting static subjects from a tripod, use the lowest native ISO you can.

Shooting a high ISO and exposing properly is better than shooting at a lower ISO, underexposing, and push processing.

Avoid ISO settings that are not native to the camera sensor, i.e. that are achieved by processing tricks in-camera and not by the sensor itself. An artificially low ISO setting, like 50/L on the 1Ds, sacrifices dynamic range. You're no better off than shooting at ISO 100 with regard to capturing highlights. An artificially high ISO, like 3200/H on the 1D-MkII is the same as underexposing and using a positive exposure compensation setting in the RAW converter.
[a href=\"index.php?act=findpost&pid=90465\"][{POST_SNAPBACK}][/a]

What is the definition of "native ISO"? What makes ISO settings "artificial"?

From my readings, a camera's lowest ISO setting means a sensor and its surrounding circuits are operating at a sensitivity (or response to light levels) that results in an optimal signal to noise ratio. Perhaps that is a good definition for "native ISO". When ISO is raised, the sensor's signal is amplified in hardware to boost the sensor's sensitivity. This comes at the expense of moving away from the sweet spot of the optimal signal to noise ratio, or at a cost of increase in noise. If the higher ISO settings are results of hardware adjustments, I would not call them "artificial". Perhaps it is not a given that all ISO settings are done by hardware, making things even more complicated.

I am by no means an expert on this, but just trying to digest in layman's terms from my readings. I think of operating at the lowest ISO is like normal film development, and raising the ISO is like pushing/pulling film development.
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John Sheehy
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« Reply #14 on: December 18, 2006, 12:18:13 PM »
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From what I understand from John's previous posts on this subject, ISO settings have traditionally moved in double numbers, ISO 100, 200, 400, 800 etc.

In the latest Canon models, we have fractions of this. ISO 250, 320, 640 etc.

These are the non-native ISOs that serve no real, purpose.
[a href=\"index.php?act=findpost&pid=91165\"][{POST_SNAPBACK}][/a]

They serve no real purpose for the RAW shooter, but they can be useful for the JPEG shooter with some cameras.  The 30D's ISOs 160/320/640/1250 are actually better than the real ISOs that they're working off of, since they are basically exposing to the right, without any danger of clipping any more than the main ISO's JPEGs would clip.
« Last Edit: December 18, 2006, 12:27:51 PM by John Sheehy » Logged
Jonathan Wienke
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« Reply #15 on: December 18, 2006, 01:49:00 PM »
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In the latest Canon models, we have fractions of this. ISO 250, 320, 640 etc.

These are the non-native ISOs that serve no real, purpose.

Absolutely NOT. What I'm referring to as the "native range" are ISO settings obtained by adjusting the voltage gain of the amplifiers between the sensor and the A/D converter before A/D conversion. "Non-native" settings are those derived by in-camera mathematical manipulations of the RAW data after A/D conversion, and typically must be enabled by some type of "ISO extension" or similar menu function before they can be selected. These "extended" settings generally offer the RAW shooter no advantage whatsoever; they can be duplicated or bettered by shooting at an ISO setting within the "native range" and push or pull processing. ISO 50/L on the 1Ds and ISO 3200/H on the 1D-MkII are examples.

The "native" ISO of the sensor itself is that which involves no voltage amplification (unity gain) between the sensor and A/D converter(s), and normally is the camera's lowest ISO setting, usually between 50 and 100.

Amplifying the voltage prior to A/D conversion is of benefit because it is scaling the output of the sensor to more-or-less match the input range of the A/D convertor, which gives you as many bits of image data as possible to work with. Manipulating the bits in-camera after A/D is of very limited use; instead of working with real (though potentially noisy) image data across the range of RAW data values, one is confining the image to a smaller range of RAW values and interpolating the image from there.
« Last Edit: December 18, 2006, 01:59:20 PM by Jonathan Wienke » Logged

John Sheehy
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« Reply #16 on: December 18, 2006, 04:55:28 PM »
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Absolutely NOT. What I'm referring to as the "native range" are ISO settings obtained by adjusting the voltage gain of the amplifiers between the sensor and the A/D converter before A/D conversion. "Non-native" settings are those derived by in-camera mathematical manipulations of the RAW data after A/D conversion, and typically must be enabled by some type of "ISO extension" or similar menu function before they can be selected. These "extended" settings generally offer the RAW shooter no advantage whatsoever; they can be duplicated or bettered by shooting at an ISO setting within the "native range" and push or pull processing. ISO 50/L on the 1Ds and ISO 3200/H on the 1D-MkII are examples.

The in-between ISOs on the 30D are made the same way, even if they are within what you call the "native range".  ISO 160 is 200 pulled one stop, then the digitized data divided by 1.26; ISO 250 is 200 pushed one stop and multiplied by 1.26.  The ISO 250 RAW histogram has a gap about every 5th value, ISO 160 has a doubly-populated value about every 4th value.  ISO 250 is missing 1/3 stop of highlights that it had from before its digitization was multiplied.

Quote
The "native" ISO of the sensor itself is that which involves no voltage amplification (unity gain) between the sensor and A/D converter(s),

The lowest ISO requires amplification, too.  Just not as much as the higher ISOs.  There is some constant, K, where ISO 100 has 1K gain, ISO 1600 16K gain, etc.

Even when signals are not amplified, they are still buffered, which is basically an amplification that just happens to be unity.

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and normally is the camera's lowest ISO setting, usually between 50 and 100.[a href=\"index.php?act=findpost&pid=91233\"][{POST_SNAPBACK}][/a]

The native base ISO of the sensor may be somewhere between 0.7k to 1.5K, but not used by the camera as such.
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Jonathan Wienke
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« Reply #17 on: December 18, 2006, 05:22:45 PM »
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The in-between ISOs on the 30D are made the same way, even if they are within what you call the "native range".  ISO 160 is 200 pulled one stop, then the digitized data divided by 1.26; ISO 250 is 200 pushed one stop and multiplied by 1.26.  The ISO 250 RAW histogram has a gap about every 5th value, ISO 160 has a doubly-populated value about every 4th value.  ISO 250 is missing 1/3 stop of highlights that it had from before its digitization was multiplied.
The lowest ISO requires amplification, too.  Just not as much as the higher ISOs.  There is some constant, K, where ISO 100 has 1K gain, ISO 1600 16K gain, etc.

Even when signals are not amplified, they are still buffered, which is basically an amplification that just happens to be unity.
The native base ISO of the sensor may be somewhere between 0.7k to 1.5K, but not used by the camera as such.

I wasn't aware of that being the case with the 30D's "between stops" ISO settings. The 1-series bodies seem to use varying voltage amplification for all ISO settings in the "native range", as opposed to firmware tricks on the RAW data after-the-fact. What tool are you using to check/verify this?
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macgyver
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« Reply #18 on: December 18, 2006, 09:08:34 PM »
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I shoot at 3200 on my 30D quite a  bit, are you saying that it isn't real 3200, but mere trickery?
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bjanes
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« Reply #19 on: December 18, 2006, 09:40:43 PM »
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I shoot at 3200 on my 30D quite a  bit, are you saying that it isn't real 3200, but mere trickery?
[{POST_SNAPBACK}][/a]

You might want to look at this analysis by [a href=\"http://http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary/]Roger Clark[/url]

In general there is not much point using an ISO much higher than that at unity gain of the sensor, which maxes out at 1600 for the Canon 5D and is probably around 1000-1200 for your camera. At unity gain one electron corresponds to one 12 bit data number in the raw file, and it does not make sense to digitize anything less. In other words, ISOs above 1600 achieve little benefit.

Bill
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