Noise Performance in an ISOless System

Started Mar 26, 2013 | Discussions
Allan Olesen Veteran Member • Posts: 3,391
Re: Noise Performance in an ISOless System

rubank wrote:

Allan Olesen wrote:

So it is wrong to say that the ISO-less method does not have any advantages to IQ. It does. That is why it is better in some scenarios.

There will always be some scenario where something other than your usual way will be better which still doesn´t make this "other" a good method in general.

No, but in an earlier post you said this:

"And to be frank, I can´t hesitate wonder where the benefit is in a routine that with some cumbersome work under certain conditions can yield the same, but not better, results than a more tradtional and straightforward routine"

I of course agree that if ISO-less can never give better results and is harder to use, there is no reason for using it.

So if you were right that it cannot give better results, you would also be right that there is not any point in using it. But I have described a scenario where it can give better results, and that means that there is a point in using it in those scenarios.

I lose the shadows - but less so when setting ISO at exposure rather than in PP. Of course my D800 gives maximum DR at base ISO, given a reasonable exposure.

If you lose shadows, you are either not operating in the ISO-less range of your camera, or your PP software is doing something bad to your raws. Lightroom 3 did this, while Lightroom 4 (Process 2012) behaves much better.

Please see my examples above using RT.

I can't explain those. But I can use circular logic, however annoying that may be:

A camera's ISO-less range (if the camera has such a range) begins at the lowest ISO which can be used instead of any higher ISO without losing any real shadow information in the raw file.

So if you have lost real shadow information in the raw file, you were not in the ISO-less range.

Steen Bay Veteran Member • Posts: 7,418
Re: Noise Performance in an ISOless System

Allan Olesen wrote:

rubank wrote:

Allan Olesen wrote:

So it is wrong to say that the ISO-less method does not have any advantages to IQ. It does. That is why it is better in some scenarios.

There will always be some scenario where something other than your usual way will be better which still doesn´t make this "other" a good method in general.

No, but in an earlier post you said this:

"And to be frank, I can´t hesitate wonder where the benefit is in a routine that with some cumbersome work under certain conditions can yield the same, but not better, results than a more tradtional and straightforward routine"

I of course agree that if ISO-less can never give better results and is harder to use, there is no reason for using it.

So if you were right that it cannot give better results, you would also be right that there is not any point in using it. But I have described a scenario where it can give better results, and that means that there is a point in using it in those scenarios.

Well, if you have the time to expose 'properly' at the 'correct' ISO, meaning that you don't blow important highlights, then ISO-less shooting won't give better results, but guess it can be useful if in a hurry, especially if the camera has an OVF. If the camera has an EVF with 'exposure simulation' (WYSIWYG) and a live histogram, then it's much easier/faster to get the 'correct' exposure.

John Sheehy Forum Pro • Posts: 22,345
Re: Noise Performance in an ISOless System

dosdan wrote:

If the camera is designed to have a proper ISOless mode, shooting this way not need be any different, including review image brightness, as far as the user is concerned. It's in the raw development phase that the extra flexibility comes into play.

Of course, this is exactly how ISOs work on medium format cameras, so it's not like the writers of converters don't know how to do the right thing already.

pavi1 Veteran Member • Posts: 6,868
Re: My name is Joseph James, and I endorse this post. : )

Well done, Dan! Combine this with gollywop's post:

http://forums.dpreview.com/forums/post/51136862

and Daniel Browning's six-part treatice:

http://forums.dpreview.com/forums/post/32064270

and you'll have a good understanding of the technical aspects of the digitally captured photo.

EDIT:  Dan's post is not meant to teach you how to take "good photos" -- it's simply to help you understand how the digital capture works, with the hope that the competent photographer might be able to improve the IQ of at least some photos in some circumstances, not unlike a race car driver understanding the mechanics of their car.  That is, you don't have to know how a car works to be a good driver, but it doesn't hurt to know why and how the car works, and sometimes helps.

dosdan wrote:

Conceptually, as far as noise is concerned, a DSLR signal path can be considered as:

Sensor -> PGA -> ADC

Each of these stages contributes noise. Beside the photonic noise inherent in light itself ("shot" noise), the most common noise is read noise. The contributions of read noise from the 3 stages above can be lumped together by combining the 3 noise components in quadrature (root-mean-square) into a value called the Total Read Noise.

However since some of these noise components are affected by ISO gain, the total read noise changes as gain is applied. It is important to understand this and its effect on overall system noise performance.

The PGA (Programmable Gain Amplifier) is used to apply stepped amounts of analogue gain (increasing the ISO sensitivity) when the signal from the sensor is weak i.e. the exposure level is low. In this discussion, I'll generally omit its noise contribution. You would hope that an amp used to reduce the total noise level was of a low-noise design itself. Some of the PGA noise will come from the input stage of the PGA, and therefore increase as you boost the analogue gain. Another part of the PGA noise will come from the output stage, so it will be of a constant level and not affected by analogue gain changes.

The sensor read noise will be boosted as the analogue gain is increased.

The ADC read noise, coming after the PGA stage, is constant in level, regardless of the amount of analogue gain.

So the simplified signal path becomes:

Sensor (analogue gain influenced) -> ADC

Since noise is coming from different parts of the signal path, the noise is either "input referenced" (photo-electrons or e-) or "output referenced" (DN - Digital Numbers, or ADU - Analogue to Digital Units).

I'll be using www.sensorgen.info data, which is input referenced. This means that the noise is considered as if it was another signal coming from the sensor. So the sensor read noise level (I'll call it just "sensor") might be 3e-. Since the ADC noise is after the PGA, we need to reverse the effect of ISO boost, if we are to think of it as another sensor signal. For example, an ADC has a read noise level of 16e-. At base ISO100 we'll consider the PGA gain to be 1x (it will probably be different from this, but we're interested in relative gain changes in this discussion). So at ISO1600 the PGA gain is 16x, and our ADC noise needs to be divided by 16. Therefore, with input-referencing, the ADC noise is seen at ISO100 as an 16e- signal from the sensor, and at ISO1600, as if it was a 1e- sensor signal. So the the input-referenced total read noise value tends to fall as ISO increases.

The formula for the simplified total read noise we'll be discussing here is:

Total = sqrt(Sensor^2 + ADC^2)

The shape of the graph of this equation vs ISO is very important. We'll be considering the ISOs of ISO100 & ISO1600. Generally, with modern sensors, we're still within the ISO range were analogue gain is applied and any in-camera raw NR is not yet enabled.

Let's look at the K-5 total read noise within this range (Manufacturers ISO used):

http://www.sensorgen.info/PentaxK-5.html

ISO Total e-
100 3.5
200 3.1
400 2.6
800 2.4
1600 1.9

The "16x" ratio (ISO1600/ISO100) of the change in the total read noise as ISO increases is 1.4:1, or just 1.4x. If you play around in a spreadsheet you can roughly get the corresponding values that produce this curve. The values are: sensor=2e- & ADC=2.8e-.

So the K-5 values are:
Sensor: 2e-
ADC: 2.8e-
16x: 1.4x

Other APS-C:

60D:
Sensor: 3e-
ADC: 13e-
16x: 4.3x

550D
Sensor: 2.7e-
ADC: 11.6e-
16x: 4.3x

7D
Sensor: 2.8e-
ADC: 8e-
16x: 2.9x

A77
Sensor: 2.7e-
ADC: 3.5e-
16x: 1.3x

D5200
Sensor: 2e-
ADC: 3e-
16x: 1.5x

FF sensors:

D600
Sensor: 3.7-
ADC: 6.3-
16x: 1.7x

D800
Sensor: 2.9e-
ADC: 3.6e-
16x: 1.2x

D4
Sensor: 2.4e-
ADC: 18.5e-
16x: 7.7x

5D III
Sensor: 4.2e-
ADC: 33e-
16x: 7.9x

1D X
Sensor: 2.6e-
ADC: 38e-
16x: 14.6x

The sensel read noise value will be affected by its size. Here we're not interested in its absolute value, but the way it contributes to the total read noise value.

The "16x" figure shows how effective the use of analogue gain is in improving the total read noise value, as the sensor output level decreases (i.e. at lower exposure levels). Sensors where this value is <2 are approaching ISOless operation. You can also get some idea how bad an idea it would be to operate a camera like the 1D X at base ISO and rely on much boosting of the rendering brightness afterwards in PP. Operating in a suitable ISO range is much better for this camera.

Once the amplified sensor read noise is much bigger then the ADC read noise, there is little reason to apply further analogue gain, as sensor read noise dominates the total read noise, and the total read noise now increases at the same rate as the amplified signal increases. So the SNR due to the read noise remains the same. Therefore for high-ISO operation, cameras switch to digital gain.

With some cameras, particularly the D600 & D800, there are difficulties in getting reasonable curve fits using just Sensor & ADC read noise values. Since the sensorgen info is derived by solving a curve fit of the DxOMark Full SNR graph, and since DxoMark is known to smooth the curves, this is possibly the reason. It could also be that PGA noise does need to be considered in some cases. Or it could be that Nikon is doing something unusual.

The 7D is unusual in having a relatively low 16x ratio compared to the other Canon cameras. It looks like the ADC read noise level is unusually low for a Canon camera.

Some camera have such low 16x values that it seems no change of analogue gain is being used at all: D7000; RX100. Perhaps the unusual results from the D800 should also be considered this way.

Note: just because a camera is ISOless, does not means it's a low-noise design. Some P&S cameras used only digital gain to change ISO. This may have been done just for simplicity.

Why worry about ISOless operation? Boosting ISO can be done in either an analogue or digital manner. Doing so digitally only improves the rendered image brightness. Using analogue gain at low ISO helps with reducing the contribution of ADC noise to the total read noise, as well as increasing the brightness of a rendered image. However both forms of gain have a negative effect: they decrease the DR (Dynamic Range).

The max. level is determined by the saturation level, either the sensel become full of photo-electrons i.e. reaching FWC (Full-Well Capacity) or reaching FS (Full Scale) in the ADC output level (running out of bits). FWC is only a consideration at base ISO. Once you need to apply extra gain (i.e. due to a low exposure level), FS clipping is the main consideration. Since FS is fixed, as you further amplify the signal, the distance between the noise floor and max. level decreases. If total read noise was constant with ISO, DR would decrease 1 stop for every doubling of gain/ISO. However, since total read noise usually changes in systems using analogue gain, DR decrease steps tend to be smaller at low ISO and increase in size, up to a max. of 1 stop per doubling of gain, at mid ISO. At high-ISO, where digital gain is used, the DR decrease should be exactly 1 step. However two thing can cause the DR value to be different.

1. High ISO involve small sensor signals do it becomes more difficult to get accurate measurements.

2. Cameras may apply raw-level NR.

The problem with decreasing DR as ISO increases, is that it also increases the likelihood of a highlight or specular reflection being clipped. If you're shooting raw, and the total read noise is relatively low, it becomes feasible to shoot at either base ISO (ISOless), or at a relatively low max. ISO, e.g. ISO400 (semi-ISOless), and then increase the rendered image brightness afterwards in PP.

So instead of, in a low-exposure situation, shooting at ISO1600, you instead shoot at ISO100 and afterwards apply 4-stops boost (16x) in PP, or in an auto-ISO setup, shoot at ISO400 (your set max. ISO), and later apply 2 stops boost in PP.

This means that relatively low levels have been recorded and it is unlikely that any highlights will be blown in the capturing phase. Of course, if you just applied the same boost in PP to reach the same rendered brightness level, you run the same chance of blowing a highlight. But since the highlight, as captured, is not yet blown, you still have an opportunity to fiddle with the tone curve to successfully render the image within the limited DR of a print or display screen. So this form of operation becomes like a superior form of highlight protection/recovery.

The disadvantage for the raw shooter is that the review image is darkish. But if cameras & raw file formats were set up to accommodate this type of operation, the review image need not be darkish at all. As the relative contribution of ADC read noise to the total read noise keeps decreasing, it is rather frustrating to see that no manufacturer has yet offered this mode.

Dan.

Nice, but I don't need to know how to drill an oil well to be able to drive a car.
--
Everything happens for a reason. #1 reason: poor planning
WSSA #44

Allan Olesen Veteran Member • Posts: 3,391
Re: Noise Performance in an ISOless System

Steen Bay wrote:

Well, if you have the time to expose 'properly' at the 'correct' ISO, meaning that you don't blow important highlights, then ISO-less shooting won't give better results, but guess it can be useful if in a hurry, especially if the camera has an OVF. If the camera has an EVF with 'exposure simulation' (WYSIWYG) and a live histogram, then it's much easier/faster to get the 'correct' exposure.

That certainly depends on your definition of "exposed properly".

What if correct exposure of the main content of the photo leads to blown highlights? Then by your definition of correct exposure, you will need to underexpose the main content to avoid highlight clipping. And if you do that, you are actually doing ISO-less photography by my definition.

Steen Bay Veteran Member • Posts: 7,418
Re: Noise Performance in an ISOless System

John Sheehy wrote:

dosdan wrote:

If the camera is designed to have a proper ISOless mode, shooting this way not need be any different, including review image brightness, as far as the user is concerned. It's in the raw development phase that the extra flexibility comes into play.

Of course, this is exactly how ISOs work on medium format cameras, so it's not like the writers of converters don't know how to do the right thing already.

Phase One IQ180 - Camera ISO - Measured ISO (DxO)

35 -- 29

50 -- 29

100 - 29

200 - 60

400 - 120

800 - 239 (etc.)

The headroom increases up to ISO 100. At ISO 100 and above Phase One has app. 1.5 stops more highlight headroom than the normal/average headroom on DSLRs.

John Sheehy Forum Pro • Posts: 22,345
Re: Noise Performance in an ISOless System

Steen Bay wrote:

John Sheehy wrote:

Of course, this is exactly how ISOs work on medium format cameras, so it's not like the writers of converters don't know how to do the right thing already.

Phase One IQ180 - Camera ISO - Measured ISO (DxO)

35 -- 29

50 -- 29

100 - 29

200 - 60

400 - 120

800 - 239 (etc.)

The headroom increases up to ISO 100. At ISO 100 and above Phase One has app. 1.5 stops more highlight headroom than the normal/average headroom on DSLRs.

I guess if you repeat something long enough, someone will make an exception to make you wrong.

Looks like they might start using arithmetic above 100, but they still have to deal with different gray points in the RAW files up to 100.

Steen Bay Veteran Member • Posts: 7,418
Re: Noise Performance in an ISOless System

Allan Olesen wrote:

Steen Bay wrote:

Well, if you have the time to expose 'properly' at the 'correct' ISO, meaning that you don't blow important highlights, then ISO-less shooting won't give better results, but guess it can be useful if in a hurry, especially if the camera has an OVF. If the camera has an EVF with 'exposure simulation' (WYSIWYG) and a live histogram, then it's much easier/faster to get the 'correct' exposure.

That certainly depends on your definition of "exposed properly".

What if correct exposure of the main content of the photo leads to blown highlights? Then by your definition of correct exposure, you will need to underexpose the main content to avoid highlight clipping. And if you do that, you are actually doing ISO-less photography by my definition.

Yes, guess you can put it like that. ETTR, while protecting important highlights, can sometimes be the same as ETTL (Exposing To The Left).

rubank Senior Member • Posts: 1,113
Re: Noise Performance in an ISOless System

Allan Olesen wrote:

rubank wrote:

Allan Olesen wrote:

So it is wrong to say that the ISO-less method does not have any advantages to IQ. It does. That is why it is better in some scenarios.

There will always be some scenario where something other than your usual way will be better which still doesn´t make this "other" a good method in general.

No, but in an earlier post you said this:

"And to be frank, I can´t hesitate wonder where the benefit is in a routine that with some cumbersome work under certain conditions can yield the same, but not better, results than a more tradtional and straightforward routine"

I of course agree that if ISO-less can never give better results and is harder to use, there is no reason for using it.

So if you were right that it cannot give better results, you would also be right that there is not any point in using it. But I have described a scenario where it can give better results, and that means that there is a point in using it in those scenarios.

You are right, I did contradict myself. I humbly apologise.
My excuse is that I said that in answer to a post that demonstrated exactly "(almost) as good but not better".

I lose the shadows - but less so when setting ISO at exposure rather than in PP. Of course my D800 gives maximum DR at base ISO, given a reasonable exposure.

If you lose shadows, you are either not operating in the ISO-less range of your camera, or your PP software is doing something bad to your raws. Lightroom 3 did this, while Lightroom 4 (Process 2012) behaves much better.

Please see my examples above using RT.

I can't explain those. But I can use circular logic, however annoying that may be:

A camera's ISO-less range (if the camera has such a range) begins at the lowest ISO which can be used instead of any higher ISO without losing any real shadow information in the raw file.

So if you have lost real shadow information in the raw file, you were not in the ISO-less range.

(Deleted my comment, it wasn´t nice)

Jack Hogan Veteran Member • Posts: 7,061
ISOless side by side example

rubank wrote:

Just to add some proof to my point of view:These images were converted using the same settings, with +4 exp comp for the ISO 100 pic.

Some mild noise reduction was used (10 L, 25 C), otherwise default medium ISO settings.
RT version is 4.0.10.36. Camera Nikon D800.

rubank, could you make the NEFs available to play with (free dropbox, google drive, skydrive account)?  Something about contrast and the texture of the snow...

Jack

Jack Hogan Veteran Member • Posts: 7,061
The shape of the curve vs ISO

dosdan wrote:

The formula for the simplified total read noise we'll be discussing here is:

Total = sqrt(Sensor^2 + ADC^2)

The shape of the graph of this equation vs ISO is very important.

Dan, can you elaborate a bit what you mean by this last statement?

Allan Olesen Veteran Member • Posts: 3,391
Re: Noise Performance in an ISOless System

rubank wrote:

Allan Olesen wrote:

So if you were right that it cannot give better results, you would also be right that there is not any point in using it. But I have described a scenario where it can give better results, and that means that there is a point in using it in those scenarios.

You are right, I did contradict myself. I humbly apologise.
My excuse is that I said that in answer to a post that demonstrated exactly "(almost) as good but not better".

People seem very polarized on this issue. Either ISO-less is the only right way to do all photography, or it is unusable in all situations.

I stand somewhere inbetween those two extremes. In my opinion, it is too badly supported by present cameras and raw converters to be allround usable. But there are two specific cases where it can be worth the effort:

  1. If the camera does not have Auto ISO in Manual (most Sony DSLRs and SLTs fit this description, including my a77), scene lighting changes too much and you do not want to set ISO for every shot, even though you want to set shutter speed and aperture manually.
  2. If there are highlights in the scene which would not be preserved at high ISO if exposing correctly for the main subject.
But I can use circular logic, however annoying that may be:

[...]

(Deleted my comment, it wasn´t nice)

I told you it would be annoying.

Great Bustard Forum Pro • Posts: 43,280
Then again...

pavi1 wrote:

Well done, Dan! Combine this with gollywop's post:

http://forums.dpreview.com/forums/post/51136862

and Daniel Browning's six-part treatice:

http://forums.dpreview.com/forums/post/32064270

and you'll have a good understanding of the technical aspects of the digitally captured photo.

EDIT:  Dan's post is not meant to teach you how to take "good photos" -- it's simply to help you understand how the digital capture works, with the hope that the competent photographer might be able to improve the IQ of at least some photos in some circumstances, not unlike a race car driver understanding the mechanics of their car.  That is, you don't have to know how a car works to be a good driver, but it doesn't hurt to know why and how the car works, and sometimes helps.

Nice, but I don't need to know how to drill an oil well to be able to drive a car.

...I imagine if the car came with a manual six speed transmission and a five speed automatic, you might want to understand the differences between the two options before purchasing.

OP dosdan Contributing Member • Posts: 537
Re: The shape of the curve vs ISO

Jack Hogan wrote:

dosdan wrote:

The formula for the simplified total read noise we'll be discussing here is:

Total = sqrt(Sensor^2 + ADC^2)

The shape of the graph of this equation vs ISO is very important.

Dan, can you elaborate a bit what you mean by this last statement?

I think it's of interest:

1. At what ISO ADC-referenced sensor RN overtakes ADC RN as the predominant read noise source.

2. At what ISO the sensor RN becomes the overwhelming source of the total RN.

ADC-Referenced Read Noises Curves for 3 APS-C & 1 FF sensor.

Dan.

 dosdan's gear list:dosdan's gear list
Pentax K-3 Pentax K-5 Pentax K20D Pentax K-01 Pentax K100D Super +1 more
Detail Man
Detail Man Forum Pro • Posts: 17,091
Re: The shape of the curve vs ISO

dosdan wrote:

Jack Hogan wrote:

dosdan wrote:

The formula for the simplified total read noise we'll be discussing here is:

Total = sqrt(Sensor^2 + ADC^2)

The shape of the graph of this equation vs ISO is very important.

Dan, can you elaborate a bit what you mean by this last statement?

I think it's of interest:

1. At what ISO ADC-referenced sensor RN overtakes ADC RN as the predominant read noise source.

2. At what ISO the sensor RN becomes the overwhelming source of the total RN.

ADC-Referenced Read Noises Curves for 3 APS-C & 1 FF sensor.

Am curious as to how that data was derived. Seems like they might be hard things to differentiate ?

OP dosdan Contributing Member • Posts: 537
Re: The shape of the curve vs ISO

Detail Man wrote:

dosdan wrote:

Jack Hogan wrote:

dosdan wrote:

The formula for the simplified total read noise we'll be discussing here is:

Total = sqrt(Sensor^2 + ADC^2)

The shape of the graph of this equation vs ISO is very important.

Dan, can you elaborate a bit what you mean by this last statement?

I think it's of interest:

1. At what ISO ADC-referenced sensor RN overtakes ADC RN as the predominant read noise source.

2. At what ISO the sensor RN becomes the overwhelming source of the total RN.

ADC-Referenced Read Noises Curves for 3 APS-C & 1 FF sensor.

Am curious as to how that data was derived. Seems like they might be hard things to differentiate ?

I used the Sensorgen data and then fiddled with my own Sensor RN & ADC noise values to get a decent curvefit.

See the RN Contributions tab here (I've only do the extra work for the 4 cameras shown above):

http://dl.dropbox.com/u/51093924/Noise%20Components%20Contribution%20v2%20.xls

Dan.

 dosdan's gear list:dosdan's gear list
Pentax K-3 Pentax K-5 Pentax K20D Pentax K-01 Pentax K100D Super +1 more
rubank Senior Member • Posts: 1,113
Re: ISOless side by side example

Jack Hogan wrote:

rubank wrote:

Just to add some proof to my point of view:These images were converted using the same settings, with +4 exp comp for the ISO 100 pic.

Some mild noise reduction was used (10 L, 25 C), otherwise default medium ISO settings.
RT version is 4.0.10.36. Camera Nikon D800.

rubank, could you make the NEFs available to play with (free dropbox, google drive, skydrive account)?  Something about contrast and the texture of the snow...

Jack

Already deleted.
Last night I did another test, low contrast low light interior with flash. ISO,s 100 vs 3200.
It didn´t come out good for the ISO 100. However, I´ve deleted these too so I can´t even show the conversions, just mention MY results.
More hearsay: several months ago someone posted very nice night sky shots from somewhere in Australia, shot at ISO 6400 with the D800.
In a comment the well known Marianne Oelund questioned why the use of ISO 6400, it would be better to underexpose at ISO below 1600.

Afte having done a few tests I fully understand why the photog used ISO 6400....

rubank Senior Member • Posts: 1,113
Re: Noise Performance in an ISOless System

Allan Olesen wrote:

rubank wrote:

Allan Olesen wrote:

So if you were right that it cannot give better results, you would also be right that there is not any point in using it. But I have described a scenario where it can give better results, and that means that there is a point in using it in those scenarios.

You are right, I did contradict myself. I humbly apologise.
My excuse is that I said that in answer to a post that demonstrated exactly "(almost) as good but not better".

People seem very polarized on this issue. Either ISO-less is the only right way to do all photography, or it is unusable in all situations.

I don´t regard it as useless in ALL situations, just in most.

I stand somewhere inbetween those two extremes. In my opinion, it is too badly supported by present cameras and raw converters to be allround usable. But there are two specific cases where it can be worth the effort:

  1. If the camera does not have Auto ISO in Manual (most Sony DSLRs and SLTs fit this description, including my a77), scene lighting changes too much and you do not want to set ISO for every shot, even though you want to set shutter speed and aperture manually.
  2. If there are highlights in the scene which would not be preserved at high ISO if exposing correctly for the main subject.

On 2: now you´re comparing underexposure at low ISO to "correct" exposure at high ISO. Nothing stops you from using some underexposure at high ISO to preserve highlight. This might still render better result than heavy underexposure at low ISO, and in my experience it MOSTLY does.

Allan Olesen Veteran Member • Posts: 3,391
Re: Noise Performance in an ISOless System

rubank wrote:

Allan Olesen wrote:

  1. If there are highlights in the scene which would not be preserved at high ISO if exposing correctly for the main subject.

On 2: now you´re comparing underexposure at low ISO to "correct" exposure at high ISO. Nothing stops you from using some underexposure at high ISO to preserve highlight. This might still render better result than heavy underexposure at low ISO, and in my experience it MOSTLY does.

If I make the same underexposure at a higher ISO, the total exposure will be lower. That creates more noise.

You are looking at this from the wrong end, so to speak. In low light, the exposure is already given: The slowest shutter speed which doesn't cause unwanted blur, and the most open aperture which doesn't cause unwanted lack of sharpness. You just have to pick the ISO which is best at catching the information in that exposure.

David Hull
David Hull Veteran Member • Posts: 6,385
Re: The shape of the curve vs ISO

I assume that what you are calling "sensor" read noise also includes the contribution from any analog amplification?  In other words it represents the noise figure of the detector cell and the AFE stage (whatever that might be) before AD conversion.

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