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

[xpatUSA]

In that case, the change in shutter speed changed the exposure and therefore the S/N ratio.

All of a sudden, I'm the bad guy in this thread!

The lower exposure gave a lower mean signal value but also and consequently a lower standard deviation. So can someone please explain to why the SNRs in that quick rough test can not be compared ??

Also can someone please define the exact exposures needed for a perfect, satisfactory test that would actually pass muster here?

Try full manual control, set the aperture and shutter speed, and change only the ISO. By doing that the exposure remains constant with the only variable being any gain supplied by ISO.

Thank you for the suggestion.

I'm curious to find out your results.

With my camera, a Sigma SD9, using the same aperture and shutter speed and changing only the ISO will give me exactly the same raw data and therefore exactly the same SNR in RawDigger. The SD9 is truly ISO-less, if I understand that term correctly, in that the ISO setting does not affect the raw data at all.

I'm no expert and maybe I'm wrong but that seems like the best approach to me.

The SD9 is a very simple camera; it only outputs a raw file and has almost zero in-camera bells and whistles unlike more modern cameras.

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

 

[dbdalrymple]

With many if not most systems, the ISO value is set by a digital input to a Programmable Gain Amplifier.

A clear assertion, I can't judge whether it is true or false.

Not really.

It is followed by an ambiguous negation.

The ISO value is set by doing a lot of things to the image.

Perhaps I should read "is implemented" for "is set"? In the vernacular the ISO value "is set" (I will not hedge with "often") by turning a dial.

You've decided to participate by revealing your own personal false assumption!

In a sub-thread discussing a variety internal implementations, it may be difficult to follow while focusing through the filter of a simple external implementation assumption.

It might (and usually does) involve a variable gain amplifier, but quite rarely is the gain of that (or those, there is often two of them)amplifier set directly according to the iSO.

"Directly" powerfully adds to the overall ambiguity. Was xpatUSA right but insufficiently precise in his formulation? It's hard to say.

In the very early days, when the processing chips in cameras were slow, 16 bit affairs without multipliers, dedicated imaging pipelines, SIMD or vector instructions, a bit of analog computing, via the use of a VGA to scale the input to the ADC was a useful thing to do.

We meander down Memory Lane. Many technical terms are mentioned. They don't clear anything up.

There's that false assumption filter at work.

More recently, variable gain has been more about optimising the use of the VGA.

The coup de grace, though no particular light is shed.

I fold.

That's what happens when you enter the twilight zone.

This exchange is what I mean by clear as milk.

Please note that I follow these threads in Open Talk with interest.

You've been able to participate without following much, but you did bring lots of milk! You do understand how these things work and know what to do to play along.

Dale B Dalrymple

 

[Chris Noble]

With my camera, a Sigma SD9, using the same aperture and shutter speed and changing only the ISO will give me exactly the same raw data and therefore exactly the same SNR in RawDigger. The SD9 is truly ISO-less, if I understand that term correctly, in that the ISO setting does not affect the raw data at all.

I'm not familiar with the SD9, but the DPReview review says it has a 12-bit A/D (narrow by today's standards) and the ISO range is only 100-400 (also quite narrow).

My guess is that almost all digital cameras would not exhibit ISO effects on the Raw file over such a narrow ISO range over just the 12 MSBs (most significant bits) of the Raw file.

(By the way the SD9 review refers to the ISO range as "Sensitivity", which will upset some of the pundits here!).

In any case, a camera like the SD9 is not really relevant to the discussion of the effect of ISO setting on SNR, as it essentially has a fixed ISO (100). "Changing" the ISO setting is indistinguishable from changing the brightness of the output image in PP in other cameras (if I understand the description of the camera's operation).

You can't really call a camera "ISO-less" if the ISO "control" has no effect on what is recorded.

 

[bobn2]

(By the way the SD9 review refers to the ISO range as "Sensitivity", which will upset some of the pundits here!).

It's not a matter of pundits being upset. It's just that the use of the word 'sensitivity' tends to inculcate wrong notions about how things work. Very often people interpret 'sensitivity' to mean that the camera becomes more sensitive to light, which it doesn't (except for the case of dual conversion gain, where the sensor genuinely becomes more sensitive with the gain step).

 

[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

There are two 'gains' at work here, which is one reason why you need to be specific about which 'gain' your talking about. The first 'gain' is a variable voltage gain stage (or, more often, two of them) at the input to the ADC. The purpose of this gain stage is to allow the full bit depth of the ADC to be used a low exposures (assumed to go along with high ISO settings). The second 'gain' is 'conversion gain', which isn't really 'gain' at all. It determines the relationship between charge in the pixel and voltage output of the pixel (which is why it sin't 'gain', different quantities in and out). This is more recent. A switchable conversion gain system was developed by Aptina (now On Semiconductor) and Sony gained access to it via a patent swap. Using this system the conversion gain is switched from low to high at one iSO setting in the range, and causes a characteristic 'step' in Bill's graphs. The reason for switching conversion gain is that it is responsible for how the downstream electronic noise looks in 'input referred' terms. That is, the higher the conversion gain the less noisy a given amount of voltage noise looks. However, a high conversion gain limits the saturation capacity of a pixel (so-called 'full well capacity' is another highly misleading term), so if you build sensors with a very high conversion gain, they will require a high base ISO. Switching the conversion gain squares this particular circle.

Thanks.

Don

 

[chrisfisheye]

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

There are two 'gains' at work here, which is one reason why you need to be specific about which 'gain' your talking about. The first 'gain' is a variable voltage gain stage (or, more often, two of them) at the input to the ADC. The purpose of this gain stage is to allow the full bit depth of the ADC to be used a low exposures (assumed to go along with high ISO settings). The second 'gain' is 'conversion gain', which isn't really 'gain' at all. It determines the relationship between charge in the pixel and voltage output of the pixel (which is why it sin't 'gain', different quantities in and out). This is more recent. A switchable conversion gain system was developed by Aptina (now On Semiconductor) and Sony gained access to it via a patent swap. Using this system the conversion gain is switched from low to high at one iSO setting in the range, and causes a characteristic 'step' in Bill's graphs. The reason for switching conversion gain is that it is responsible for how the downstream electronic noise looks in 'input referred' terms. That is, the higher the conversion gain the less noisy a given amount of voltage noise looks. However, a high conversion gain limits the saturation capacity of a pixel (so-called 'full well capacity' is another highly misleading term), so if you build sensors with a very high conversion gain, they will require a high base ISO. Switching the conversion gain squares this particular circle.

Whatever (for photography purpose).

ISO can change the gain to improve SNR. You absolutely do not need to know which type of gain is applied.

Anyway, different camera makers could make different choices and apply different level of analog/digital amplifications ( know you will not like this terminology). Fuji for instance makes very different imllementation choices than other manufacturer though they use the same sensor...

The only "important" information is how SNR increases with ISO. The internals are not that important. Of course you can be curious !!! But I do not see any misconceptions at all with the different kind of analog gains that can be applied.

I used double with "important" because you do not have to care so much about it. With low exposure just try to use high ISO with whatever camera and you will benefit from the SNR advantage.

 

[Chris Noble]

(By the way the SD9 review refers to the ISO range as "Sensitivity", which will upset some of the pundits here!).

It's not a matter of pundits being upset. It's just that the use of the word 'sensitivity' tends to inculcate wrong notions about how things work. Very often people interpret 'sensitivity' to mean that the camera becomes more sensitive to light, which it doesn't (except for the case of dual conversion gain, where the sensor genuinely becomes more sensitive with the gain step).

No, it doesn't in that case either. Same sensor (same wells, same photon-gathering ability), but with two different analog gain paths in parallel.

 

[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

There are two 'gains' at work here, which is one reason why you need to be specific about which 'gain' your talking about. The first 'gain' is a variable voltage gain stage (or, more often, two of them) at the input to the ADC. The purpose of this gain stage is to allow the full bit depth of the ADC to be used a low exposures (assumed to go along with high ISO settings). The second 'gain' is 'conversion gain', which isn't really 'gain' at all. It determines the relationship between charge in the pixel and voltage output of the pixel (which is why it sin't 'gain', different quantities in and out). This is more recent. A switchable conversion gain system was developed by Aptina (now On Semiconductor) and Sony gained access to it via a patent swap. Using this system the conversion gain is switched from low to high at one iSO setting in the range, and causes a characteristic 'step' in Bill's graphs. The reason for switching conversion gain is that it is responsible for how the downstream electronic noise looks in 'input referred' terms. That is, the higher the conversion gain the less noisy a given amount of voltage noise looks. However, a high conversion gain limits the saturation capacity of a pixel (so-called 'full well capacity' is another highly misleading term), so if you build sensors with a very high conversion gain, they will require a high base ISO. Switching the conversion gain squares this particular circle.

Whatever (for photography purpose).

ISO can change the gain to improve SNR. You absolutely do not need to know which type of gain is applied.

Don't tell people what they "need to know". It's up to each person, in a free country, to decide that for himself or herself.

Anyway, different camera makers could make different choices and apply different level of analog/digital amplifications ( know you will not like this terminology). Fuji for instance makes very different imllementation choices than other manufacturer though they use the same sensor...

The only "important" information is how SNR increases with ISO. The internals are not that important. Of course you can be curious !!! But I do not see any misconceptions at all with the different kind of analog gains that can be applied.

I used double with "important" because you do not have to care so much about it. With low exposure just try to use high ISO with whatever camera and you will benefit from the SNR advantage.

 

[bobn2]

(By the way the SD9 review refers to the ISO range as "Sensitivity", which will upset some of the pundits here!).

It's not a matter of pundits being upset. It's just that the use of the word 'sensitivity' tends to inculcate wrong notions about how things work. Very often people interpret 'sensitivity' to mean that the camera becomes more sensitive to light, which it doesn't (except for the case of dual conversion gain, where the sensor genuinely becomes more sensitive with the gain step).

No, it doesn't in that case either. Same sensor (same wells, same photon-gathering ability), but with two different analog gain paths in parallel.

That's not what dual conversion gain is. It's an extra capacitor in the pixel circuit which can be switched in and out. The voltage output for the pixel is given by V = Q/C. The smaller the capacitance the larger the output voltage for a given charge. Properly sensitivity does not mean how big is the output for a given input (which is another reason 'sensitivity' is the wrong term), it relates to what is the size of the smallest stimulus that can be detected. Dual conversion gain reduces read noise when the capacitance is set low, thus smaller stimuli can be detected. It's a genuine sensitivity increase.

What' you're talking about could be called 'dual channel gain' Which has a high and low gain channel to two ADCs (or one taking two samples in succession) and then the two ADC samples are combined. It's a technique that actually originated on these forums, I believe, and was the idea of Professor Emil Martinec, who used to post here regularly.

 

[bobn2]

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

There are two 'gains' at work here, which is one reason why you need to be specific about which 'gain' your talking about. The first 'gain' is a variable voltage gain stage (or, more often, two of them) at the input to the ADC. The purpose of this gain stage is to allow the full bit depth of the ADC to be used a low exposures (assumed to go along with high ISO settings). The second 'gain' is 'conversion gain', which isn't really 'gain' at all. It determines the relationship between charge in the pixel and voltage output of the pixel (which is why it sin't 'gain', different quantities in and out). This is more recent. A switchable conversion gain system was developed by Aptina (now On Semiconductor) and Sony gained access to it via a patent swap. Using this system the conversion gain is switched from low to high at one iSO setting in the range, and causes a characteristic 'step' in Bill's graphs. The reason for switching conversion gain is that it is responsible for how the downstream electronic noise looks in 'input referred' terms. That is, the higher the conversion gain the less noisy a given amount of voltage noise looks. However, a high conversion gain limits the saturation capacity of a pixel (so-called 'full well capacity' is another highly misleading term), so if you build sensors with a very high conversion gain, they will require a high base ISO. Switching the conversion gain squares this particular circle.

Whatever (for photography purpose).

ISO can change the gain to improve SNR. You absolutely do not need to know which type of gain is applied.

Oh, you absolutely do. The different types of gain have quite different effects.

 

[Chris Noble]

ISO can change the gain to improve SNR. You absolutely do not need to know which type of gain is applied.

Oh, you absolutely do. The different types of gain have quite different effects.

Agreed. DIgital ISO is like the brightness slider of your PP software.

 

[chrisfisheye]

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

There are two 'gains' at work here, which is one reason why you need to be specific about which 'gain' your talking about. The first 'gain' is a variable voltage gain stage (or, more often, two of them) at the input to the ADC. The purpose of this gain stage is to allow the full bit depth of the ADC to be used a low exposures (assumed to go along with high ISO settings). The second 'gain' is 'conversion gain', which isn't really 'gain' at all. It determines the relationship between charge in the pixel and voltage output of the pixel (which is why it sin't 'gain', different quantities in and out). This is more recent. A switchable conversion gain system was developed by Aptina (now On Semiconductor) and Sony gained access to it via a patent swap. Using this system the conversion gain is switched from low to high at one iSO setting in the range, and causes a characteristic 'step' in Bill's graphs. The reason for switching conversion gain is that it is responsible for how the downstream electronic noise looks in 'input referred' terms. That is, the higher the conversion gain the less noisy a given amount of voltage noise looks. However, a high conversion gain limits the saturation capacity of a pixel (so-called 'full well capacity' is another highly misleading term), so if you build sensors with a very high conversion gain, they will require a high base ISO. Switching the conversion gain squares this particular circle.

Whatever (for photography purpose).

ISO can change the gain to improve SNR. You absolutely do not need to know which type of gain is applied.

Don't tell people what they "need to know". It's up to each person, in a free country, to decide that for himself or herself.

What ???? Are you serious ?

How can you interpret what I wrote the way you did ?

It is not very respectful.

 

[Chris Noble]

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

Analog does not necessarily mean continuous.

An analog signal-processing step can operate in discrete step settings.

Analog ISO control operates in discrete steps that correspond to the ISO values. It would be continuous if your ISO control was a continuous adjustment (like a volume control knob on a radio, although in modern circuits, even that is digital but with extremely fine steps).

Discussions get complicated because sometimes the subject is the underlying signal, and other times its about how that signal is controlled. An analog amplifier can be controlled digitally but it's still operating on an analog signal.

"Analog ISO" refers to ISO changes that are applied to the sensor signal before the analog-to-digital conversion, and digital ISO after. Digital ISO is like the brightness slider on your PP software.

 

[Chris Noble]

Very often people interpret 'sensitivity' to mean that the camera becomes more sensitive to light, which it doesn't (except for the case of dual conversion gain, where the sensor genuinely becomes more sensitive with the gain step).

No, it doesn't in that case either. Same sensor (same wells, same photon-gathering ability), but with two different analog gain paths in parallel.

That's not what dual conversion gain is. It's an extra capacitor in the pixel circuit which can be switched in and out.

I stand corrected! Thank you.

 

[xpatUSA]

With my camera, a Sigma SD9, using the same aperture and shutter speed and changing only the ISO will give me exactly the same raw data and therefore exactly the same SNR in RawDigger. The SD9 is truly ISO-less, if I understand that term correctly, in that the ISO setting does not affect the raw data at all.

I'm not familiar with the SD9, but the DPReview review says it has a 12-bit A/D (narrow by today's standards) and the ISO range is only 100-400 (also quite narrow).

My guess is that almost all digital cameras would not exhibit ISO effects on the Raw file over such a narrow ISO range over just the 12 MSBs (most significant bits) of the Raw file.

(By the way the SD9 review refers to the ISO range as "Sensitivity", which will upset some of the pundits here!).

Ahhh, yes ... not unlike most descriptions of the dreaded 'Exposure Triangle' where we get told how ISO alters the sensitivity of the sensor !!

In any case, a camera like the SD9 is not really relevant to the discussion of the effect of ISO setting on SNR, as it essentially has a fixed ISO (100). "Changing" the ISO setting is indistinguishable from changing the brightness of the output image in PP in other cameras (if I understand the description of the camera's operation).

Thank you for your assessment of a camera model that I have been using for almost ten years. I'm not sure that telling me how you think that an SD9 works is helping my sub-discussion with my worthy correspondent.

May I remind you that initially neither he nor I mentioned a particular camera model; and he has no gear list; ergo, his statement that increasing the ISO setting increases SNR was a general statement to the forum - even though he may have been talking about his own camera, as was I.

You can't really call a camera "ISO-less" if the ISO "control" has no effect on what is recorded.

OK. What is the definition of ISO-less? Would prefer a credible reference rather than an opinion.

[edit] found this:

"An ISO invariant sensor will produce the same results in terms of image quality if you underexpose your photograph and then raise the exposure in post-processing as if you had exposed it correctly (in camera) in the first place."

https://capturetheatlas.com/iso-invariance/ [/edit]

Can't agree with the use of "exposure" in that context, but we all know what is really meant, eh?

You're right: the SD9 is certainly not "ISO-less", duh. As indeed was proved by my earlier example. I must stop using the term to describe the many Sigma digital cameras that brighten the raw during conversion.

I have said many times that the concept of "ISO" for digital cameras is misleading compared to film and I still wish that it would have been called something else ...

... like "inverse SNR", for example ... ;-)

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

 

[xpatUSA]

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

Analog does not necessarily mean continuous.

An analog signal-processing step can operate in discrete step settings.

Analog ISO control operates in discrete steps that correspond to the ISO values. It would be continuous if your ISO control was a continuous adjustment (like a volume control knob on a radio, although in modern circuits, even that is digital but with extremely fine steps).

Discussions get complicated because sometimes the subject is the underlying signal, and other times its about how that signal is controlled. An analog amplifier can be controlled digitally but it's still operating on an analog signal.

"Analog ISO" refers to ISO changes that are applied to the sensor signal before the analog-to-digital conversion, and digital ISO after. Digital ISO is like the brightness slider on your PP software.

Hmmm ... once upon a time I had access to an Analog Computer as an electronic controls ehgineer - not a "discrete step" to be seem anywhere ... all pots, capacitors and operational amplifiers ... LOL.

 

[bobn2]

You can't really call a camera "ISO-less" if the ISO "control" has no effect on what is recorded.

OK. What is the definition of ISO-less? Would prefer a credible reference rather than an opinion.

I'm quite likely the person that coined the term 'ISOless', unless anyone can find an earlier example than this . There isn't a formal definition, but a working one would be a camera which has a constant read noise over its whole ISO range.

For those that are interested there were a few threads thereafter.

This one from Pierre Sottas. And this one that I started, discussing how a camera without an ISO control would be designed. Then this one , which put about some more detailed ideas. Unfortunately they are all sufficiently old that most of the image links fail, but all the text is still there.

--
Is it always wrong
for one to have the hots for
Comrade Kim Yo Jong?

 

[robert1955]

You can't really call a camera "ISO-less" if the ISO "control" has no effect on what is recorded.

OK. What is the definition of ISO-less? Would prefer a credible reference rather than an opinion.

I'm quite likely the person that coined the term 'ISOless', unless anyone can find an earlier example than this . There isn't a formal definition, but a working one would be a camera which has a constant read noise over its whole ISO range.

That seems a sharper / narrower definition than current usage which is of the free lunch kind

For those that are interested there were a few threads thereafter.

This one from Pierre Sottas.

I think you meant this one:

https://www.dpreview.com/forums/thread/2903658#forum-post-36887819

And this one that I started, discussing how a camera without an ISO control would be designed. Then this one , which put about some more detailed ideas. Unfortunately they are all sufficiently old that most of the image links fail, but all the text is still there.

 

[bobn2]

You can't really call a camera "ISO-less" if the ISO "control" has no effect on what is recorded.

OK. What is the definition of ISO-less? Would prefer a credible reference rather than an opinion.

I'm quite likely the person that coined the term 'ISOless', unless anyone can find an earlier example than this . There isn't a formal definition, but a working one would be a camera which has a constant read noise over its whole ISO range.

That seems a sharper / narrower definition than current usage which is of the free lunch kind

Which is why looking for a formal definition is somewhat futile.

For those that are interested there were a few threads thereafter.

This one from Pierre Sottas.

I think you meant this one:

https://www.dpreview.com/forums/thread/2903658#forum-post-36887819

Oops. Thanks for that. Looking back at it it's fascinating how exactly the same conversation can carry on for twelve years. If you look through those threads they contain everything that's been said in this thread, just about. The exception is dual conversion gain, which wasn't around then. One of the interesting bits was the discussion of Emil Martinec's dual channel gain idea, and the lamenting that Canon hadn't implemented it in their cameras. Well, fast forward twelve years and they have.

 

[Chris Noble]

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

Analog does not necessarily mean continuous.

An analog signal-processing step can operate in discrete step settings.

Analog ISO control operates in discrete steps that correspond to the ISO values. It would be continuous if your ISO control was a continuous adjustment (like a volume control knob on a radio, although in modern circuits, even that is digital but with extremely fine steps).

Discussions get complicated because sometimes the subject is the underlying signal, and other times its about how that signal is controlled. An analog amplifier can be controlled digitally but it's still operating on an analog signal.

"Analog ISO" refers to ISO changes that are applied to the sensor signal before the analog-to-digital conversion, and digital ISO after. Digital ISO is like the brightness slider on your PP software.

Hmmm ... once upon a time I had access to an Analog Computer as an electronic controls ehgineer - not a "discrete step" to be seem anywhere ... all pots, capacitors and operational amplifiers ... LOL.

Yes, technology has advanced in the past 50 years! WOL ;-) Analog is in microchips as much as digital, and there is a lot of digitally-controlled analog signal processing.