Why do we still use analog gain with ISO invariant sensors?

[bodeswell]

1. samcd wrote:

There's certainly a trade-off, and there is another benefit to one analog gain and digitization, too; compressed RAW files are much smaller when most of the most significant bits are unused. That would make higher-ISO RAW files smaller; not larger.

I'm not sure which way round you mean this. If read-chain gain is increased at high ISO settings the lower bits get filled with (useless) noise, which impedes compression. If the upper bits are just zeros (no variable gain) they will compress well.

In the end, the ISO control is probably redundant. The camera processor has all the information that it needs to optimally pack the sensor information into the available ADC width, varying the read chain gain if needs be. The JFIF engine has all the information that it needs from the raw file to make a rendering. At most, ISO should be a cue to help signal the photographers intent, though I think that a rendering intent control would be better. Auto ISO is a kind of poor effort at that.

Discussions of digital camera ISO always seem to follow the same story arc on DPR. A naïf asks a question that incorporates a false assumption.

I'm not sure that naïf is entirely appropriate here. When we talk about 'analog gain' and 'ISO invariant sensors' we're already well into the abstruse from the get-go. These are fringe topics in the photography community, so if you want to just stick to the mainstream, don't choose to participate in threads of this kind.

My naïf self has definitely appreciated this discussion! In particular, this was very interesting and basically the answer I was looking for:

"In the end, the ISO control is probably redundant. The camera processor has all the information that it needs to optimally pack the sensor information into the available ADC width, varying the read chain gain if needs be."

There is an initial clarifying response or two, but then gradually the discussion becomes more and more abstruse. After a while the whole topic becomes clear as milk.

That's about it. That's the nature of the topic.

For example, the damnable control is redundant, something entirely different is needed, and it’s not clear what that is. It isn’t clear that there is any helpful guidance for the innocent in the meantime.

One of the nice things about forums is discussion. Someone puts a half-formed idea out there, other people contribute and in the end, people learn and sometimes even innovate. I think that's great. For some people it's outside their comfort zone, but participation is not compulsory.

Alas, you were not the naïf I had in mind. No one in this thread was. I was thinking about many other threads over the years.

With respect to the answer that you were looking for and got, let me ask this question: what practical maxim(s) would you derive from it?

Good question. I'm always interested in understanding things better even if there aren't obvious practical consequences. But still:

- Cameras' implementations of ISO are more mysterious than I had thought, so it's worth testing this on my own camera rather than trying to generalize.

- If it is the case that my camera can get a lot more highlight room with very little loss of shadow detail by shooting at base ISO when auto ISO would choose a higher one, I'll ... well I'm not sure what I'll do! Perhaps worry less about "underexposure" in low light and curse Sony for an ISO implementation that makes me choose between a dark image preview and optimal use of the camera's dynamic range!

Good answers!

 

[chrisfisheye]

1. samcd wrote:

There's certainly a trade-off, and there is another benefit to one analog gain and digitization, too; compressed RAW files are much smaller when most of the most significant bits are unused. That would make higher-ISO RAW files smaller; not larger.

I'm not sure which way round you mean this. If read-chain gain is increased at high ISO settings the lower bits get filled with (useless) noise, which impedes compression. If the upper bits are just zeros (no variable gain) they will compress well.

In the end, the ISO control is probably redundant. The camera processor has all the information that it needs to optimally pack the sensor information into the available ADC width, varying the read chain gain if needs be. The JFIF engine has all the information that it needs from the raw file to make a rendering. At most, ISO should be a cue to help signal the photographers intent, though I think that a rendering intent control would be better. Auto ISO is a kind of poor effort at that.

Discussions of digital camera ISO always seem to follow the same story arc on DPR. A naïf asks a question that incorporates a false assumption.

I'm not sure that naïf is entirely appropriate here. When we talk about 'analog gain' and 'ISO invariant sensors' we're already well into the abstruse from the get-go. These are fringe topics in the photography community, so if you want to just stick to the mainstream, don't choose to participate in threads of this kind.

My naïf self has definitely appreciated this discussion! In particular, this was very interesting and basically the answer I was looking for:

"In the end, the ISO control is probably redundant. The camera processor has all the information that it needs to optimally pack the sensor information into the available ADC width, varying the read chain gain if needs be."

There is an initial clarifying response or two, but then gradually the discussion becomes more and more abstruse. After a while the whole topic becomes clear as milk.

That's about it. That's the nature of the topic.

For example, the damnable control is redundant, something entirely different is needed, and it’s not clear what that is. It isn’t clear that there is any helpful guidance for the innocent in the meantime.

One of the nice things about forums is discussion. Someone puts a half-formed idea out there, other people contribute and in the end, people learn and sometimes even innovate. I think that's great. For some people it's outside their comfort zone, but participation is not compulsory.

Alas, you were not the naïf I had in mind. No one in this thread was. I was thinking about many other threads over the years.

With respect to the answer that you were looking for and got, let me ask this question: what practical maxim(s) would you derive from it?

Good question. I'm always interested in understanding things better even if there aren't obvious practical consequences. But still:

- Cameras' implementations of ISO are more mysterious than I had thought, so it's worth testing this on my own camera rather than trying to generalize.

- If it is the case that my camera can get a lot more highlight room

In auto exposure modes, with ev=0, you have as much highlight room so no mre risks than usual to clip highlights.

with very little loss of shadow detail by shooting at base ISO

Little loss is still a loss

when auto ISO would choose a higher one, I'll ... well I'm not sure what I'll do! Perhaps worry less about "underexposure" in low light and curse Sony for an ISO implementation that makes me choose between a dark image preview and optimal use of the camera's dynamic range!

This is not necessarily optimal, it is only potential dynamic range !

Just shoot normally and simply avoid to clip highlights. You may use lower ISO for this but you will use lower ISO only when necessary. You get the most of your sensor in every case !.

Using a low ISO systematically is not a strategy which is not really necessary in my opinion. If this is 100% isoless, why not.

Good answers!

 

[Chris Noble]

I'm old-school. Variable analog gain has much lower noise than any sensor or A/D and the delay is zero. Not bad.

I agree. It's in most cases a better engineering solution than a wider ADC. I'm not a fan of the term 'analog gain'. You're not gaining analogs, and 'gain' itself is an analog concept, applied to whatever quantity is being 'gained' (in the case of a camera VGA it's voltage). In the digital domain, if you make a number larger it's called multiplication.

Analog gain and digital gain were very commonly used terms when I did electronic design, but that was decades ago. However I just did a Google search and they are still commonly used by circuit designers.

I've never heard the terms "gaining analogs" or "gaining a quantity" though.

As I said, you're not gaining analogs, which was the point. On the quantity, electronic engineers will talk about voltage gain or current gain and occasionally charge gain. That's 'gaining a quantity', the quantity being voltage, current or charge.

And gain can be greater or less than unity in an electronic circuit.

Yes, they are. Within a discipline people tend to have their own framework of jargon. Electronic engineers are well accustomed to talking about 'gain', but the will say in general which kind of gain it is (unless it's obvious by contexts) so they'll talk about 'current gain' or 'voltage gain'. When digital circuits began to be incorporated into analog systems, they they found that digital multiplication operators could provide the same function as could 'gain', so they called it 'digital gain'.

In the world of computer science things were different. Circuits that performed multiplication were called 'multipliers'. For analog computing the variable gain amplifiers that did this function were still called 'multipliers' (and still are).

Photography is neither electronic engineering or computer science, so if we adopt those communities' terminology without understanding then we confuse ourselves. I say this with a fair amount of confidence, because I am an electronic engineer, a computer scientist and a photographer.

The reason I'm concerned about terminology is that poor use can lead thought patterns down a garden path. That's the case here. If we consider a camera as a black box, it takes light in at one end and puts out perceptual specifications at the other. It doesn't emit light. Inside the black box a translation is made from the input to the output, and gain is no part of that translation. There is no reason why any arbitrary amount of light might not be translated to any arbitrary lightness (lightness being the component of that perceptual specification which says how light or dark something should look). Internal 'gains' are as irrelevant to the essence that conversion as are the details of the computer code used to do it. So, you do not need to invoke 'gain' to explain how a smaller amount of light translates to a lighter image. And doing so sometimes leads to erroneous thought patterns. People logically assume that 'gain' means that something is being 'gained', and then the thing that is gained is either light or some unspecified analog to light they they often call 'signal'.

In most digital cameras, ISO control is effected over most of its range by a variable analog gain stage between the sensor and the A/D. It's a pretty simple concept, and variable gain between a sensor and an A/D is a common circuit in many applications. I speak as a photographer and also someone who used to design data-conversion circuits.

 

[Autonerd]

If we consider a camera as a black box, it takes light in at one end and puts out perceptual specifications at the other. It doesn't emit light.

Well, it could if it's an old film camera and the light seals are sufficiently worn...



Aaron

 

[bobn2]

In most digital cameras, ISO control is effected over most of its range by a variable analog gain stage between the sensor and the A/D. It's a pretty simple concept, and variable gain between a sensor and an A/D is a common circuit in many applications. I speak as a photographer and also someone who used to design data-conversion circuits.

I would quibble with the exact choice of words. ISO control is not 'effected' by a variable 'analog' (actually voltage) stage. Rather, the control of ISO includes a change of gain over some of the ISO range. But that's not the point really. The control of ISO includes different operations, very often camera dependent. To focus on one, which is not universal, and say that ISO 'is' that one, and use it as an explanation, when it is largely irrelevant to what the ISO control actually does is just misleading. People who are not electronic engineers don't know what 'gain' is in any detail, so it's just swapping one unknown concept for another. Along with it will come misconceptions, such as that so-called 'ISO noise' is cause by 'amplification', which, if you have designed data conversion circuits, you will know is false.

 

[D Cox]

I'm old-school. Variable analog gain has much lower noise than any sensor or A/D and the delay is zero. Not bad.

I agree. It's in most cases a better engineering solution than a wider ADC. I'm not a fan of the term 'analog gain'. You're not gaining analogs, and 'gain' itself is an analog concept, applied to whatever quantity is being 'gained' (in the case of a camera VGA it's voltage). In the digital domain, if you make a number larger it's called multiplication.

Analog gain and digital gain were very commonly used terms when I did electronic design, but that was decades ago. However I just did a Google search and they are still commonly used by circuit designers.

I've never heard the terms "gaining analogs" or "gaining a quantity" though.

As I said, you're not gaining analogs, which was the point. On the quantity, electronic engineers will talk about voltage gain or current gain and occasionally charge gain. That's 'gaining a quantity', the quantity being voltage, current or charge.

And gain can be greater or less than unity in an electronic circuit.

Yes, they are. Within a discipline people tend to have their own framework of jargon. Electronic engineers are well accustomed to talking about 'gain', but the will say in general which kind of gain it is (unless it's obvious by contexts) so they'll talk about 'current gain' or 'voltage gain'. When digital circuits began to be incorporated into analog systems, they they found that digital multiplication operators could provide the same function as could 'gain', so they called it 'digital gain'.

In the world of computer science things were different. Circuits that performed multiplication were called 'multipliers'. For analog computing the variable gain amplifiers that did this function were still called 'multipliers' (and still are).

Photography is neither electronic engineering or computer science, so if we adopt those communities' terminology without understanding then we confuse ourselves. I say this with a fair amount of confidence, because I am an electronic engineer, a computer scientist and a photographer.

The reason I'm concerned about terminology is that poor use can lead thought patterns down a garden path. That's the case here. If we consider a camera as a black box, it takes light in at one end and puts out perceptual specifications at the other. It doesn't emit light. Inside the black box a translation is made from the input to the output, and gain is no part of that translation. There is no reason why any arbitrary amount of light might not be translated to any arbitrary lightness (lightness being the component of that perceptual specification which says how light or dark something should look). Internal 'gains' are as irrelevant to the essence that conversion as are the details of the computer code used to do it. So, you do not need to invoke 'gain' to explain how a smaller amount of light translates to a lighter image. And doing so sometimes leads to erroneous thought patterns. People logically assume that 'gain' means that something is being 'gained', and then the thing that is gained is either light or some unspecified analog to light they they often call 'signal'.

In most digital cameras, ISO control is effected over most of its range by a variable analog gain stage between the sensor and the A/D. It's a pretty simple concept, and variable gain between a sensor and an A/D is a common circuit in many applications. I speak as a photographer and also someone who used to design data-conversion circuits.

Is the gain continuously variable, or is it switched from low to high at a certain ISO number ? Bill Claff's graphs suggest the latter, and so do the occasional nuggets of information released by camera companies.

Don

 

[xpatUSA]

<>

Wouldn't we be better off if, beyond their second base ISO, cameras just baked an exposure adjustment into the exif info instead of applying more analog gain? So if "proper" image brightness required ISO 1600, the camera would instead use its highest base ISO (say, ISO 400) and then tell Lightroom to start at +2 on the exposure slider. Compared to using ISO 1600 we'd get more highlight protection and about the same noise.

Why do cameras not do this? Is there a problem I'm not seeing?

Late to the thread and have nothing to offer that has not already been said in various ways - apart from the fact that many Sigma Foveon-based camera models do actually "do this"!

For a given light value and exposure setting (by which I mean aperture and shutter NOT including ISO) the raw data is invariant with respect to the ISO setting. The ISO setting is sent in meta-data to the converter software which brightens the image accordingly during conversion.

The Sigma camera that I use has ISO settings of 100, 200 or 400 with no in-betweens. All very simple with no ifs, buts or mebbes.

Stuff shot at 100 ISO and -2 EC is just as crappy as the same stuff shot at 400 ISO and 0 EC because the raw data is identical in each case i.e. it is truly invariant ... ;-)

--
what you got is not what you saw ...

 

[Chris Noble]

In most digital cameras, ISO control is effected over most of its range by a variable analog gain stage between the sensor and the A/D. It's a pretty simple concept, and variable gain between a sensor and an A/D is a common circuit in many applications. I speak as a photographer and also someone who used to design data-conversion circuits.

I would quibble with the exact choice of words. ISO control is not 'effected' by a variable 'analog' (actually voltage) stage.

Not just voltage; a lot of current gain as well due to the impedance mismatch.

Rather, the control of ISO includes a change of gain over some of the ISO range. But that's not the point really. The control of ISO includes different operations, very often camera dependent. To focus on one, which is not universal, and say that ISO 'is' that one, and use it as an explanation, when it is largely irrelevant to what the ISO control actually does is just misleading. People who are not electronic engineers don't know what 'gain' is in any detail, so it's just swapping one unknown concept for another. Along with it will come misconceptions, such as that so-called 'ISO noise' is cause by 'amplification', which, if you have designed data conversion circuits, you will know is false.

I fully agree that these concepts are challenging and largely irrelevant to practicing photographers, but the concept and execution of variable signal gain between a sensor and an A/D are basic processes for experienced electronic engineers. That does not disparage photographers; electronics is a profession, like surgery and accounting, and those last two are just as mysterious to me as electronics is to those who have not done electronics design.

 

[Chris Noble]

In most digital cameras, ISO control is effected over most of its range by a variable analog gain stage between the sensor and the A/D. It's a pretty simple concept, and variable gain between a sensor and an A/D is a common circuit in many applications. I speak as a photographer and also someone who used to design data-conversion circuits.

Is the gain continuously variable, or is it switched from low to high at a certain ISO number ? Bill Claff's graphs suggest the latter, and so do the occasional nuggets of information released by camera companies.

Don

Since the ISO control is not continuous, there is no reason for the ISO gain to be either. On the other hand, doubling the gain for every doubling of ISO over most of the range seems to match the data, and is not a complicated circuit.

 

[Chris Noble]

For a given light value and exposure setting (by which I mean aperture and shutter NOT including ISO) the raw data is invariant with respect to the ISO setting. The ISO setting is sent in meta-data to the converter software which brightens the image accordingly during conversion.

That is not the case for any of my Lumix cameras. Changing the ISO setting shifts the Raw data, which can easily be confirmed by looking at the Raw histogram. Furthermore, it looks like an analog gain change before the A/D, because there are no missing codes (an artifact of digital gain).

 

[xpatUSA]

For a given light value and exposure setting (by which I mean aperture and shutter NOT including ISO) the raw data is invariant with respect to the ISO setting. The ISO setting is sent in meta-data to the converter software which brightens the image accordingly during conversion.

That is not the case for any of my Lumix cameras.

Nor any of mine. Missing from the above quote is the context of my Sigma camera. The quoted statement was never intended to apply to all cameras.

Changing the ISO setting shifts the [Lumix] Raw data, which can easily be confirmed by looking at the Raw histogram. Furthermore, it looks like an analog gain change before the A/D, because there are no missing codes (an artifact of digital gain).

None of which negates anything I said in the context of my post. Some Sigma cameras do have AFEs to which your comment above would apply.

--
what you got is not what you saw ...

 

[Chris Noble]

With a fixed shutter speed and aperture, there are two ways we can brighten an image:

- by using a higher ISO (analog gain)

- by increasing the exposure slider in post (digital gain)

My understanding is that most modern sensors are approximately ISO invariant (beyond a second base ISO, if they have one), meaning that these two approaches will produce very similar noise levels in the final image.

But the digital gain approach retains the maximum amount of highlight room, while every additional stop of ISO decreases highlight room by a stop. As a result, by using ISO instead of digital gain we could lose multiple stops of dynamic range without seeing any real noise improvement.

Wouldn't we be better off if, beyond their second base ISO, cameras just baked an exposure adjustment into the exif info instead of applying more analog gain? So if "proper" image brightness required ISO 1600, the camera would instead use its highest base ISO (say, ISO 400) and then tell Lightroom to start at +2 on the exposure slider. Compared to using ISO 1600 we'd get more highlight protection and about the same noise.

Why do cameras not do this? Is there a problem I'm not seeing?

The sensor has no way of "knowing" what comes after it and how ISO control is effected. The sensor is not affected by the ISO control.

The stage that is affected by the ISO control is the A/D. If the A/D has sufficient ENOB (effective number of bits) to capture all of the data from the sensor, it is ISO invariant.

So we should really be talking about cameras with ISO-invariant digital conversion.

 

[chrisfisheye]

With a fixed shutter speed and aperture, there are two ways we can brighten an image:

- by using a higher ISO (analog gain)

- by increasing the exposure slider in post (digital gain)

My understanding is that most modern sensors are approximately ISO invariant (beyond a second base ISO, if they have one), meaning that these two approaches will produce very similar noise levels in the final image.

But the digital gain approach retains the maximum amount of highlight room, while every additional stop of ISO decreases highlight room by a stop. As a result, by using ISO instead of digital gain we could lose multiple stops of dynamic range without seeing any real noise improvement.

Wouldn't we be better off if, beyond their second base ISO, cameras just baked an exposure adjustment into the exif info instead of applying more analog gain? So if "proper" image brightness required ISO 1600, the camera would instead use its highest base ISO (say, ISO 400) and then tell Lightroom to start at +2 on the exposure slider. Compared to using ISO 1600 we'd get more highlight protection and about the same noise.

Why do cameras not do this? Is there a problem I'm not seeing?

The sensor has no way of "knowing" what comes after it and how ISO control is effected. The sensor is not affected by the ISO control.

The stage that is affected by the ISO control is the A/D. If the A/D has sufficient ENOB (effective number of bits) to capture all of the data from the sensor, it is ISO invariant.

So we should really be talking about cameras with ISO-invariant digital conversion.

The A/D comes with the sensor. It is implicit when we rate a sensor that we rate the system sensor + A/D. Besides, I am not sure we can really decouple the sensor and A/D, otherwise I would choose the best sensor coupled with the best A/D. But you are more expert than I am, do not hesitate to tell me if I am missing something.

For a photographer anyway this can be considered as a black box without the need to differentiate the sensor and the A/D. What I need to know is that by raising ISO, it can improve SNR by whatever means, no need to go into details.

 

[xpatUSA]

With a fixed shutter speed and aperture, there are two ways we can brighten an image:

- by using a higher ISO (analog gain)

- by increasing the exposure slider in post (digital gain)

My understanding is that most modern sensors are approximately ISO invariant (beyond a second base ISO, if they have one), meaning that these two approaches will produce very similar noise levels in the final image.

But the digital gain approach retains the maximum amount of highlight room, while every additional stop of ISO decreases highlight room by a stop. As a result, by using ISO instead of digital gain we could lose multiple stops of dynamic range without seeing any real noise improvement.

Wouldn't we be better off if, beyond their second base ISO, cameras just baked an exposure adjustment into the exif info instead of applying more analog gain? So if "proper" image brightness required ISO 1600, the camera would instead use its highest base ISO (say, ISO 400) and then tell Lightroom to start at +2 on the exposure slider. Compared to using ISO 1600 we'd get more highlight protection and about the same noise.

Why do cameras not do this? Is there a problem I'm not seeing?

The sensor has no way of "knowing" what comes after it and how ISO control is effected. The sensor is not affected by the ISO control.

The stage that is affected by the ISO control is the A/D. If the A/D has sufficient ENOB (effective number of bits) to capture all of the data from the sensor, it is ISO invariant.

So we should really be talking about cameras with ISO-invariant digital conversion.

The A/D comes with the sensor.

If "comes with" means that they are on the same chip, the statement is not generally correct.

It is implicit when we rate a sensor that we rate the system sensor + A/D.

Not universally implicit. For one of my cameras I can read the sensor data sheet and the A/D data sheet separately and thereby rate each component individually.

Besides, I am not sure we can really decouple the sensor and A/D, otherwise I would choose the best sensor coupled with the best A/D. But you are more expert than I am, do not hesitate to tell me if I am missing something.

On one of my cameras, the sensor data sheet makes no mention of an ADC or an A/D at all.

For a photographer anyway this can be considered as a black box without the need to differentiate the sensor and the A/D.

For myself, I prefer to think "inside the box".

What I need to know is that by raising ISO, it can improve SNR by whatever means, no need to go into details.

Without going into details, none of my cameras gain an improved SNR just by raising ISO.

Please pardon my pedantry ...

--
what you got is not what you saw ...

 

[chrisfisheye]

With a fixed shutter speed and aperture, there are two ways we can brighten an image:

- by using a higher ISO (analog gain)

- by increasing the exposure slider in post (digital gain)

My understanding is that most modern sensors are approximately ISO invariant (beyond a second base ISO, if they have one), meaning that these two approaches will produce very similar noise levels in the final image.

But the digital gain approach retains the maximum amount of highlight room, while every additional stop of ISO decreases highlight room by a stop. As a result, by using ISO instead of digital gain we could lose multiple stops of dynamic range without seeing any real noise improvement.

Wouldn't we be better off if, beyond their second base ISO, cameras just baked an exposure adjustment into the exif info instead of applying more analog gain? So if "proper" image brightness required ISO 1600, the camera would instead use its highest base ISO (say, ISO 400) and then tell Lightroom to start at +2 on the exposure slider. Compared to using ISO 1600 we'd get more highlight protection and about the same noise.

Why do cameras not do this? Is there a problem I'm not seeing?

The sensor has no way of "knowing" what comes after it and how ISO control is effected. The sensor is not affected by the ISO control.

The stage that is affected by the ISO control is the A/D. If the A/D has sufficient ENOB (effective number of bits) to capture all of the data from the sensor, it is ISO invariant.

So we should really be talking about cameras with ISO-invariant digital conversion.

The A/D comes with the sensor.

If "comes with" means that they are on the same chip, the statement is not generally correct.

I just mean they are not totally independent.

It is implicit when we rate a sensor that we rate the system sensor + A/D.

What is meant by "rate"?

To give a sensor rating like dxomarks

Besides, I am not sure we can really decouple the sensor and A/D, otherwise I would choose the best sensor coupled with the best A/D. But you are more expert than I am, do not hesitate to tell me if I am missing something.

On one of my cameras, the sensor data sheet makes no mention of an ADC or an A/D at all.

For a photographer anyway this can be considered as a black box without the need to differentiate the sensor and the A/D.

For myself, I prefer to think "inside the box".

What I need to know is that by raising ISO, it can improve SNR by whatever means, no need to go into details.

Without going into details, none of my cameras gain an improved SNR just by raising ISO.

Do you mean they are 100% ISOless ?? Really ?

The main use of ISO for me as a raw shooter is to use it to improve SNR

Please pardon my pedantry ...

 

[xpatUSA]

With a fixed shutter speed and aperture, there are two ways we can brighten an image:

- by using a higher ISO (analog gain)

- by increasing the exposure slider in post (digital gain)

My understanding is that most modern sensors are approximately ISO invariant (beyond a second base ISO, if they have one), meaning that these two approaches will produce very similar noise levels in the final image.

But the digital gain approach retains the maximum amount of highlight room, while every additional stop of ISO decreases highlight room by a stop. As a result, by using ISO instead of digital gain we could lose multiple stops of dynamic range without seeing any real noise improvement.

Wouldn't we be better off if, beyond their second base ISO, cameras just baked an exposure adjustment into the exif info instead of applying more analog gain? So if "proper" image brightness required ISO 1600, the camera would instead use its highest base ISO (say, ISO 400) and then tell Lightroom to start at +2 on the exposure slider. Compared to using ISO 1600 we'd get more highlight protection and about the same noise.

Why do cameras not do this? Is there a problem I'm not seeing?

The sensor has no way of "knowing" what comes after it and how ISO control is effected. The sensor is not affected by the ISO control.

The stage that is affected by the ISO control is the A/D. If the A/D has sufficient ENOB (effective number of bits) to capture all of the data from the sensor, it is ISO invariant.

So we should really be talking about cameras with ISO-invariant digital conversion.

The A/D comes with the sensor.

If "comes with" means that they are on the same chip, the statement is not generally correct.

I just mean they are not totally independent.

It is implicit when we rate a sensor that we rate the system sensor + A/D.

What is meant by "rate"?

To give a sensor rating like dxomarks

Besides, I am not sure we can really decouple the sensor and A/D, otherwise I would choose the best sensor coupled with the best A/D. But you are more expert than I am, do not hesitate to tell me if I am missing something.

On one of my cameras, the sensor data sheet makes no mention of an ADC or an A/D at all.

For a photographer anyway this can be considered as a black box without the need to differentiate the sensor and the A/D.

For myself, I prefer to think "inside the box".

What I need to know is that by raising ISO, it can improve SNR by whatever means, no need to go into details.

Without going into details, none of my cameras gain an improved SNR just by raising ISO.

Do you mean they are 100% ISOless ?? Really ?

No, I did not say that ... really.

The main use of ISO for me as a raw shooter is to use it to improve SNR

Good luck with that ... ;-)

 

[chrisfisheye]

With a fixed shutter speed and aperture, there are two ways we can brighten an image:

- by using a higher ISO (analog gain)

- by increasing the exposure slider in post (digital gain)

My understanding is that most modern sensors are approximately ISO invariant (beyond a second base ISO, if they have one), meaning that these two approaches will produce very similar noise levels in the final image.

But the digital gain approach retains the maximum amount of highlight room, while every additional stop of ISO decreases highlight room by a stop. As a result, by using ISO instead of digital gain we could lose multiple stops of dynamic range without seeing any real noise improvement.

Wouldn't we be better off if, beyond their second base ISO, cameras just baked an exposure adjustment into the exif info instead of applying more analog gain? So if "proper" image brightness required ISO 1600, the camera would instead use its highest base ISO (say, ISO 400) and then tell Lightroom to start at +2 on the exposure slider. Compared to using ISO 1600 we'd get more highlight protection and about the same noise.

Why do cameras not do this? Is there a problem I'm not seeing?

The sensor has no way of "knowing" what comes after it and how ISO control is effected. The sensor is not affected by the ISO control.

The stage that is affected by the ISO control is the A/D. If the A/D has sufficient ENOB (effective number of bits) to capture all of the data from the sensor, it is ISO invariant.

So we should really be talking about cameras with ISO-invariant digital conversion.

The A/D comes with the sensor.

If "comes with" means that they are on the same chip, the statement is not generally correct.

I just mean they are not totally independent.

It is implicit when we rate a sensor that we rate the system sensor + A/D.

What is meant by "rate"?

To give a sensor rating like dxomarks

Besides, I am not sure we can really decouple the sensor and A/D, otherwise I would choose the best sensor coupled with the best A/D. But you are more expert than I am, do not hesitate to tell me if I am missing something.

On one of my cameras, the sensor data sheet makes no mention of an ADC or an A/D at all.

For a photographer anyway this can be considered as a black box without the need to differentiate the sensor and the A/D.

For myself, I prefer to think "inside the box".

What I need to know is that by raising ISO, it can improve SNR by whatever means, no need to go into details.

Without going into details, none of my cameras gain an improved SNR just by raising ISO.

Do you mean they are 100% ISOless ?? Really ?

No, I did not say that ... really.

Yes you did or you just want to play with words.

The main use of ISO for me as a raw shooter is to use it to improve SNR

Good luck with that ... ;-)

Lol. I usually use high ISO in low light and it improves a bit the image,

So yes it works despite what you pretend.

I suspect you will play with words again. Anyway,,,

 

[xpatUSA]

The main use of ISO for me as a raw shooter is to use it to improve SNR

Good luck with that ... ;-)

Lol. I usually use high ISO in low light and it improves a bit the image,

So yes it works despite what you pretend.

I suspect you will play with words again. Anyway,,,

I just shot a dark corner in my room and checked the results with RawDigger ...

100 ISO: SNR 12.78

400 ISO: SNR 7.38

LOL no improvement there ... again, quite the opposite ... minus 0.8EV ...

... and no playing with words either. Just cold, hard numbers. :-D

--
what you got is not what you saw ...

 

[Bob]

I don't agree with the your assumption that a camera's ISO setting is an analog adjustment.

 

[Chris Noble]

The A/D comes with the sensor.

If "comes with" means that they are on the same chip, the statement is not generally correct.

The sensor can be integrated with the A/D on the same chip, (Panasonic has done it and I'm sure others), but you take a performance hit as the optimal semiconductor process is different for these very different types of circuit.

Especially with video performance requirements, you need fast, low-noise, pretty-high resolution (12-16 bit) A/Ds, and also sensors with pretty deep wells. Very different semiconductor processing.