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ISO what?

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#1 skround

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Posted 11 April 2021 - 07:18 PM

I'm trespassing from the Equipment part of CN as I think I'm finally able to articulate a quandary I've been having for some time.

 

ISO - what is it, at camera electronic level? Sounds like a banal question but I mean it at deeper level - so switch off your school bus:)

 

Two-line background: thinking about the rest of the triptych it's quite easy: aperture and exposure are physically modulating the amount of light landing on a sensor.

Also, in the old days it was - I think- the grain size or otherwise characteristic of the reacting medium on the film - again something physical I can grasp.

 

So what is ISO in a DSLR? Is it 'just' the electronic gain? Does it take place at sensor level or at the microprocessor level?

 

Does 'native ISO' makes any sense at all as a term?

 

Is the max ISO of a DSLR simply how much it's tolerated a grainy/noisy image thus being a loose measure of the noise quality of a camera?

 

Ok, forget the last question...

Cheers

 

 

 



#2 piaras

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Posted 11 April 2021 - 08:55 PM

“So what is ISO in a DSLR? Is it 'just' the electronic gain? Does it take place at sensor level or at the microprocessor level?”

Yes and at the processor level as far as I understand it. The chip is just a holder of pixels which is a converter of photos to an electronic unit that the microprocessor measures to determine how many photos hit the individual pixels.

Pierre


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#3 SteveInNZ

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Posted 12 April 2021 - 02:28 AM

The "S" in ISO stands for Standardization. The intention is to provide a measure of sensitivity to the user that is standard across cameras. So for a given level of light,  a Kodak camera from the 1900's with 100 ISO film, a Sony A7s or a Canon 450D set to 100 ISO will all provide a correctly exposed image with the same shutter speed and aperture (focal ratio) setting. It doesn't specify if the image looks nice or is even usable. Just that it is exposed correctly.

 

How the manufacturer achieves that under the hood, is up to them. In the case of a digital sensor, they may apply amplification or offsets in the analog (sensor) domain or the digital (microprocessor) domain or a mixture of both. With a digital sensor, the consequence of increasing the gain to achieve a higher ISO value is noise. The manufacturer will decide how much noise is tolerable for their market and limit the maximum ISO accordingly.

 

With film, the most obvious (common) consequence of increasing the ISO is the need for larger crystals which we see as grain. Again, the manufacturer will decide how much grain they can tolerate and will weigh that against other consequences like color shift, shelf life, reciprocity failure, etc.

 

Steve.


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#4 whwang

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Posted 12 April 2021 - 02:58 AM

With a digital sensor, the consequence of increasing the gain to achieve a higher ISO value is noise. 

 

Precisely speaking, it's exactly the opposite.  Higher ISO is less noisy.  See this:

 

6D_ISO.jpg

 

All were taken with Canon 6D within half an hour under identical conditions.  All have exposure time of 30 sec.  The only difference is ISO.  (The images were stretched in Photoshop to reach equal apparent brightness.  Since all of them received the same amount of signal, stretching them to have equal brightness is entirely reasonable.)  You can see that ISO 100 is the noisiest, while ISO 1600 and 3200 are the cleanest.

 

You get the impression of high ISO = high noise because in daylight photography, your exposure time is dictated by the exposure meter of the camera under the influence of your ISO setting.  If you set a higher ISO, the exposure meter will tell you to decrease the exposure time, and this leads to less signal.  The noisy images under high ISO in daylight photography are a consequence of less signal, not a consequence of higher noise.  From purely noise's point of view, higher ISO gives less noise, and this is perfectly demonstrated by these example images.


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#5 skround

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Posted 12 April 2021 - 01:17 PM

Precisely speaking, it's exactly the opposite.  Higher ISO is less noisy.  See this:

 

 

 

All were taken with Canon 6D within half an hour under identical conditions.  All have exposure time of 30 sec.  The only difference is ISO.  (The images were stretched in Photoshop to reach equal apparent brightness.  Since all of them received the same amount of signal, stretching them to have equal brightness is entirely reasonable.)  You can see that ISO 100 is the noisiest, while ISO 1600 and 3200 are the cleanest.

 

You get the impression of high ISO = high noise because in daylight photography, your exposure time is dictated by the exposure meter of the camera under the influence of your ISO setting.  If you set a higher ISO, the exposure meter will tell you to decrease the exposure time, and this leads to less signal.  The noisy images under high ISO in daylight photography are a consequence of less signal, not a consequence of higher noise.  From purely noise's point of view, higher ISO gives less noise, and this is perfectly demonstrated by these example images.

That's a very good point and it makes good sense with low light  long exposures. Still, I tend to think that there's a sweet-spot on the way to max ISO - 102800 for the extended Canon 6D. Would you agree? Did you run that test yourself?



#6 Alen K

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Posted 12 April 2021 - 04:21 PM

Precisely speaking, it's exactly the opposite.  Higher ISO is less noisy.  See this:

 

 

All were taken with Canon 6D within half an hour under identical conditions.  All have exposure time of 30 sec.  The only difference is ISO.  (The images were stretched in Photoshop to reach equal apparent brightness.  Since all of them received the same amount of signal, stretching them to have equal brightness is entirely reasonable.)  You can see that ISO 100 is the noisiest, while ISO 1600 and 3200 are the cleanest.

 

You get the impression of high ISO = high noise because in daylight photography, your exposure time is dictated by the exposure meter of the camera under the influence of your ISO setting.  If you set a higher ISO, the exposure meter will tell you to decrease the exposure time, and this leads to less signal.  The noisy images under high ISO in daylight photography are a consequence of less signal, not a consequence of higher noise.  From purely noise's point of view, higher ISO gives less noise, and this is perfectly demonstrated by these example images.

Have you run this test on any other cameras, such as your Pentax 645Z? It would be interesting to see the result. On cameras with lower read noise, the results at different ISOs would be much closer, right? 



#7 whwang

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Posted 12 April 2021 - 08:15 PM

I ran similar tests on various cameras that I own and invited a trusted friend to run tests on the cameras that I don’t have. The example above from 6D is from my friend, since I don’t have a 6D. My own tests on D800 and 645z show similar results, except that the difference between high and low ISO is smaller since these non-Canon cameras are more “ISO-less.” (So, Alen, you are right.) Canon cameras, on the other hand, have very strong ISO-dependent noise. That’s why I chose to show the 6D example here.

Based on the example above, I would say the optimal ISO for 6D for deep-sky photography is 1600. You don’t gain much (or any) by going to 3200. So there is no need to sacrifice dynamic range and use ISO above 1600 on 6D.

I show the 6D example here because it’s dramatic, and amateur people tend to only believe when they see real pictures. On the other hand, this kind of ISO-dependent noise (higher noise at low ISO) is very well documented and measured on pretty much every camera model (on Bill Claff’s website, for example). So there is no need to conduct your own tests. Just look at the read noise curves on Bill Claff’s website and you get the full picture of what’s going on. There you can decide what’s the optimal ISO for your camera.
To be clear, I meant this page: https://www.photonst...Charts/RN_e.htm

Hope this helps.

Cheers,
Wei-Hao
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#8 jpengstrom

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Posted 12 April 2021 - 08:48 PM

Precisely speaking, it's exactly the opposite.  Higher ISO is less noisy.  See this:

[Snip]

(The images were stretched in Photoshop to reach equal apparent brightness.

How were the images stretched in PS?  Adjusting the exposure until it looked the same or until the brightness values of selected pixels was the same?



#9 whwang

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Posted 12 April 2021 - 09:36 PM

How were the images stretched in PS?  Adjusting the exposure until it looked the same or until the brightness values of selected pixels was the same?

Hi,

 

This is very easy.  Just change the exposure setting in camera raw, with steps inversely proportional to ISO.  This naturally lands every image to equal apparent brightness quite precisely.  Of course, this is only possible if one shoots raw.


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#10 sharkmelley

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Posted 13 April 2021 - 01:29 AM

Precisely speaking, it's exactly the opposite.  Higher ISO is less noisy.  See this:

 

6D_ISO.jpg

 

 

There's something wrong with this link.

 

Mark



#11 whwang

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Posted 13 April 2021 - 01:52 AM

There's something wrong with this link.

 

Mark

 

 

Screen Shot 2021-04-13 at 2.50.18 PM.jpg

 

Can't you see it?



#12 crackout

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Posted 13 April 2021 - 02:37 AM

Here's a very good read on the topic: http://dslr-astropho...trophotography/

My 80D is one of the 'newer' Canon models with a more or less iso-invariant sensor.
Once ISO is of no concern, you can focus on maximizing dynamic range. For my 80D this seems to be ISO100-200.

However, for my circumstances, this dictates my exposures to be at least 40s to swamp read noise.

Edited by crackout, 13 April 2021 - 02:38 AM.

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#13 sharkmelley

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Posted 13 April 2021 - 03:00 AM

I can see it now, in your screenshot but not in your earlier post.  Here's a screenshot of what I see:

 

Capture.JPG

 

Mark


Edited by sharkmelley, 13 April 2021 - 03:01 AM.


#14 bclaff

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Posted 13 April 2021 - 09:57 AM

...

ISO - what is it, at camera electronic level? ...

...

So what is ISO in a DSLR? Is it 'just' the electronic gain? Does it take place at sensor level or at the microprocessor level?

Does 'native ISO' makes any sense at all as a term?

...

On most cameras the ISO setting is implemented using variable gain between the pixel and the Analog to Digital Converter (ADC).
That gain is increased in proportion to the ISO setting.

The pixel itself has either one or two sensitivities depending on whether it is dual conversion gain or not.

The base ISO setting is the highest ISO setting that uses the minimum amount of gain.

Increasing the gain (raising the ISO setting) improves the Signal to Noise Ratio (SNR) since only the noise upstream from the amplifier is amplified.
This is why input-referred read noise drops as you raise the ISO setting.
But, increasing the ISO setting also limits the maximum signal that can be collected which lowers the maximum SNR.
This is why dynamic range drops as you raise the ISO setting.


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#15 skround

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Posted 17 April 2021 - 10:23 AM

To be clear, I meant this page: https://www.photonst...Charts/RN_e.htm

Thanks for the link - very useful.

 

It gives real data to enter the equation to figure out the optimized sub-exposure time and gain. 

https://www.youtube....93UvP358&t=721s




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