What's different when ISO setting is changed?

[ajax]

I'm a bit of a newcomer to digital photography but old enough to have done some photography using film. As I recall, in the case of film, ISO was a property of film that indicated the speed of the film. Slower film, with lower ISO value, was more fine grained which meant that resulting photos could be sharper whereas faster film, with higher ISO value, was courser meaning that pictures may not be as sharp as with with slower film. In that, there were trade-offs involved in choosing to shoot with faster film.

I've done some web browsing trying to learn how my digital camera works. This includes reading articles such as this one about sensors. I'm having a hard time grasping how the ISO concept, intended to adjust a camera for different films (i.e., sensors), applies to the sensor in my digital camera which is always the same. The instructions for using my camera as well as various articles on this subject suggest that the concept is the same. This sort of implies that the camera can, somehow, alter the sensor. However, some limited testing with my new camera does not seem to support the idea of such an analogy with film based photography. I accept that my experiments may not be sufficient to be conclusive but I'd very much like to learn what actually happens in the camera when ISO settings change. This especially involves knowing if the camera can alter the sensor and if so how? It also seems plausible that the camera can computationally alter the data captured by the sensor in a way that relates to the film speed concepts. Maybe there is another explanation or some combination of such methods.

[Manfred M]

Welcome to CiC! If you wouldn't mind clicking on the "My Profile" button at the top of this page and adding at least your first name and country you are from? We are a fairly informal group here and go on a first name basis.

To answer your question, the camera sensor has a single "base" sensitivity, which is the lowest ISO numeric value you can use. That tends to be in the ISO 100 or ISO 200 range, although some higher end cameras do have lower settings. All other ISO values are achieved by amplifying this signal, so much like with film cameras, the lowest ISO settings tend to result in the highest quality images.

As the gain (amplification) is increased, the dynamic range drops, the digital noise level increase and the colour bit depth decreases.

[pschlute]

The other thing to note is that with film a jump from 100 to 800 ISO would usually deliver a significantly grainier picture. Digital cameras are improving very quickly their ability to cope with the digital noise that higher amplification produces. So depending on what model digital camera you own, the equivalent in graininess from 100 to 800 on film could well equate today with a jump from 100 to 3200 as far as noisiness is concerned.

[Ken MT]

Try reading through this article on ISO myths etc by Roger Clark
http://www.clarkvision.com/articles/iso/

[NorthernFocus]

...I'd very much like to learn what actually happens in the camera when ISO settings change. This especially involves knowing if the camera can alter the sensor and if so how? It also seems plausible that the camera can computationally alter the data captured by the sensor in a way that relates to the film speed concepts. Maybe there is another explanation or some combination of such methods.

As Manfred explained, when the ISO setting is increased above the base value(camera specific but usually 100-200) the "signal" is amplified. The problem with amplifying the signal is that signal "noise" increases while color and dynamic range decrease. Digital noise manifests itself much the same as film grain(i.e. image detail suffers). So the overall analogy to film is valid. Higher ISO degrades image quality. By definition quality is subjective so there are many opinions on the level of degradation.

One reason that you may not be seeing the effects of high digital ISO as much as a comparable increase in film speed is due to the signal processing that takes place inside the camera and/or in post processing software. As was mentioned in a post above the manufacturers are getting very good at improving how the signal from the sensor is processed to generate the digital image.

The bottom line of all of the above is that, yes, increasing digital ISO setting has a SIMILAR effect as shooting higher film speeds but is not a one for one comparison.

[DanK]

One reason that you may not be seeing the effects of high digital ISO as much as a comparable increase in film speed is due to the signal processing that takes place inside the camera and/or in post processing software.

This points to an important distinction. You (Ajax) didn't say whether you are shooting raw or in-camera JPEGs. If the latter, the camera is doing the postprocessing, and most cameras have postprocessing algorithms that include noise reduction (at the cost of lessened detail), so you won't notice as large an effect of boosting ISO. If you shoot raw, noise reduction is left to you, and if you don't use it, you will see the decreased quality from increased ISO when you first open the image in the raw processor.

[ajax]

Very helpful responses. Many thanks! Also lots of very nice photographs. My compliments to all.

[inkista]

One more reference link about newer sensor technology that makes some sensors (mostly or kinda/sorta) iso-invariant. So the whole "more noise with a higher ISO setting" thing isn't necessarily true any more.

https://photographylife.com/iso-invariance-explained

[DanK]

One more reference link about newer sensor technology that makes some sensors (mostly or kinda/sorta) iso-invariant. So the whole "more noise with a higher ISO setting" thing isn't necessarily true any more

I don't believe this is correct. My understanding was that ISO invariance does not mean that there is no additional noise from boosting ISO. It means that it makes no difference whether one brightens the image by amplifying the signal with an ISO increase or in post-processing. This is also what the article you linked says:

If your camera sensor is perfectly ISO invariant, there is no penalty in noise if you brighten a photo in post-production rather than increasing your ISO in-camera.

Amplification has to amplify everything the sensor has recorded, including noise.

[William W]

. . . [I am] old enough to have done some photography using film. As I recall, in the case of film, ISO was a property of film that indicated the speed of the film. Slower film, with lower ISO value, was more fine grained which meant that resulting photos could be sharper whereas faster film, with higher ISO value, was courser meaning that pictures may not be as sharp as with slower film. In that, there were trade-offs involved in choosing to shoot with faster film. . . I'm having a hard time grasping how the ISO concept, intended to adjust a camera for different films (i.e., sensors), applies to the sensor in my digital camera which is always the same.

You have nailed it.

It is a "concept", not a reality.

Arguably, it was easier to transfer a concept of what used to be and apply it to what is now - and that is exactly what happened - my guess is that the TR&D people, influenced by the Marketing Team, decided that it was easier to term this thing on a Digital Camera "ISO" so it would have a conceptual relationship with "Film Speed", rather than conjuring a new term such as, for example IARSB 'Image Amplification Relative to Sensor Base'.

WW

[NorthernFocus]

...my guess is that the TR&D people, influenced by the Marketing Team, decided that it was easier to term this thing on a Digital Camera "ISO" so it would have a conceptual relationship with "Film Speed", rather than conjuring a new term such as, for example IARSB 'Image Amplification Relative to Sensor Base'.

Or simply calling it "gain" like the rest of the electronics/instrumentation industry.

Similarly the term "electronic shutter" is an oxymoron that is used when in fact there is no shutter involved. But the mechanical shutter is electronically(i.e.servo) activated.

But without the ISO reference how would us old farts apply the sunny 16 rule?

[xpatUSA]

Or simply calling it "gain" like the rest of the electronics/instrumentation industry.

Similarly the term "electronic shutter" is an oxymoron that is used when in fact there is no shutter involved. But the mechanical shutter is electronically(i.e.servo) activated.

But without the ISO reference how would us old farts apply the sunny 16 rule?

Excellent. I've suggested alternative names for the ISO Knob along similar lines occasionally, Dan.

My favorite is "noise" 1-11 in honor of Spinal Tap ...

Another was "under-exposure" but it was too long and wouldn't please Marketing.

[DanK]

IARSB 'Image Amplification Relative to Sensor Base'

I like this. That's exactly what an increased ISO is. Just think how many explanations could have been skipped if this had been the term used....

[pschlute]

Similarly the term "electronic shutter" is an oxymoron that is used when in fact there is no shutter involved. But the mechanical shutter is electronically(i.e.servo) activated.

Except where the Electronic shutter mode is actually a EFCS. In that case the exposure is started electronically but ended by the second curtain.

[Dave Humphries]

"Gain" works for me too, but then with a previous career involving TV cameras, that's hardly a surprise.

Although "dB" to "stops" for exposure might be an alternative source of confusion

[Manfred M]

"Gain" works for me too, but then with a previous career involving TV cameras, that's hardly a surprise.

Although "dB" to "stops" for exposure might be an alternative source of confusion

My video camera shows gain in dB as well. I suspect that photographers can be a bit resistant to change, so ISO is something they can handle. As the ISO values actually follow the ISO standards, so far as I understand it, this is as good as any other means of expressing gain so long as people are aware of the "side effects" of increasing gain.

This is not a lot different than in the film days as faster film tended to be grainy and colour rendition as not as good either.

[mikesan]

As the gain (amplification) is increased, ....... the colour bit depth decreases.

Can you elaborate a bit on how this manifests itself? Perhaps cite some reference which explains it more in detail.

Many thanks.

[William W]

Or simply calling it "gain" like the rest of the electronics/instrumentation industry.

and all others' comments about the same . . .

I hear you all.

I have had this conversation in my head many times.

Seriously - probably "gain" and "db" were on the Engineers' short list, but . . . the 'influence by the Marketing Team' would not have allowed any of that - that's my conclusion.

WW

[xpatUSA]

Originally Posted by Manfred M

As the gain (amplification) is increased, ....... the colour bit depth decreases.

Can you elaborate a bit on how this manifests itself? Perhaps cite some reference which explains it more in detail.

Many thanks.

I too would like to know more.

It probably depends on what is meant by "colour bit depth" ...

[Manfred M]

Can you elaborate a bit on how this manifests itself? Perhaps cite some reference which explains it more in detail.

Many thanks.

Mike - head over to DxO Mark and look up the one of the camera models they have tested and look at their charts.

https://www.dxomark.com/