Why do digital cameras need iso?

[xpatUSA]

Good afternoon everyone, and thanks for having a go at explaining it!
I'm one of those who really likes to know how something works . .
Having gone through the great replies here it would seem that that an analogy would be a microphone and amplifier to capture sound.

Does that sound about right?

Cheers.

You're almost there, Clive. If you add into your analogy the concept of going deaf, that might get you a little closer. You see, deafness is caused not so much by the loudness of sound but exposure to it over time. By exposure, I mean the loudness of the sound multiplied by the time during which the sound occurred. With sound, the time is measured in months or years and, unlike a pixel, it's all a bit non-linear and sporadic in occurrence. But the analogy stands well if, for example, you consider bangs as photons and years as hundredths of seconds. Considering thus, you should be able to imagine the pixel output as rising with continued exposure and it's final output as being like the accumulated deafness of a person. The usual analogy for that is rain drops falling into a bucket - see the excellent tutorial on this very site.

Your analogy with amplification is perfect for the ISO part. Imagine that you are eaves-dropping on a street conversation from some distance. You can't hear them, they're too far away (i.e. not enough light). So you crank the volume (increase the ISO) and you can hear them but the traffic (noise) is quite audible too. If they were closer (brighter) but not the traffic (noise) you'd be able to turn down the volume (ISO) and still hear them but with less noise.

Your analogy goes further: you can consider the traffic noise as "photon noise" and the amplifier hiss as the pixel's "dark current" both of which are made relatively less as you turn down the volume (select a lower ISO setting).

good luck with your quest for knowledge.

[cliveg]

Thanks Ted - its now all making sense, and hopefully will lead on to better pictures (one day!).
Best wishes
Clive

[Graystar]

WHAT A GREAT QUESTION!!! Did I miss something or has there not been a SIMPLE answer?

Here's your simple answer.

The aperture and shutter allow light into the camera. But you cannot have a photograph without a response to that light. ISO defines that response. Whether it's film or digital, a response to light must still be defined. As such, ISO is still required for digital.

Film's physical reaction to light varies with ISO, and so is said to have a sensitivity to light.

A sensor's reaction to light is always the same. The signal is then amplified to provide varying responses. When the response is measured as an input-output gain, it is a responsivity to light.

http://en.wikipedia.org/wiki/Responsivity

[cliveg]

Cheers Graystar, that will do for me!

As a result of this and other comments on this thread, this was starting to become my understanding of it. So much obliged to you all.

I did check the Wiki link. It says:-
One simple expression for the responsivity R of a photodetector in which an optical signal is converted into an electrical current (known as a photocurrent) is:-

R=\eta\frac{q}{hf}\approx\eta\frac{\lambda_{(\mu m)}}{1.23985(\mu m\times W/A)}

Don't you just love mathematicians!!

[Manfred M]

As it happens, current generation digital sensors do always shoot at the base ISO of the system, and insert a shift identifier in the raw data file to mark the gain to be applied for the ISO the user requested. But that is a very recent development. It used to be the case that the dynamic range of the sensor chips wasn't high enough to allow that sort of operation, so there was an amplifier stage that would increase the ISO of the acquired data. But now, with as much as 14 EV dynamic range in the sensor, the amplifier is no longer needed, and leaving it out not only lowers costs but also removes a major source of electronic noise in the camera circuitry.

So far as I understand CMOS sensors, this statement is not correct. These sensors are active sensors, i.e. any signal must be amplified before the output is high enough for additional processing to be done. These amplifiers are fairly basic and are built into every photodetector site to minimize noise. Amplifiers, especially fairly simple ones are not linear across their entire range, so the base ISO is likely to be the point where the amplifier has just entered the (relatively) linear portion of the curve. Take a look at the DxO sensor graphs, for example http://www.dxomark.com/index.php/Pub...or-performance in a perfect world, all these graphs would be linear, and notice how in general we start seeing a bit of a s-curve at the extreme left and right sides of the graphs.

In some ways no one has addressed the other part of what ISO does it standardizes how sensor sensitivity (exposure index) are measured. This ensures us that shooting the same scene at the same ISO setting, shutter speed and f-stop, will give us the same image, regardless of camera model or manufacturer.

[davidedric]

Here's my very simple, "how I think about it" non-technical view, FWIW

With film cameras, we used to have only two usable variables: aperture and shutter speed, with the third, film speed, locked in for whatever film you had in the camera. Once cameras had some reasonable electronics you could let the camera decide both (Program), fix one or the other (Av, Tv), or manage both (Manual). Of course, on Manual you had to take care of the exposure.

Now all three variables are available to us, so now we can let the camera do all three (Program/Auto mode and Auto ISO), or any combination of fixing one, two or all three of the variables.

Each variable has it's own characteristics (like the ones for ISO that have been well described in this thread), so it's really just a case of deciding which ones matter most for the image that I am trying to make (and which compromises I'm prepared to make), and letting the camera take care of the rest - or telling me if it can't!. Or controlling all three, of course.

I'm sure the details vary from camera to camera, but that's my idiot's guide to me.

[Graystar]

Here's my very simple, "how I think about it" non-technical view, FWIW

With film cameras, we used to have only two usable variables: aperture and shutter speed, with the third, film speed, locked in for whatever film you had in the camera. Once cameras had some reasonable electronics you could let the camera decide both (Program), fix one or the other (Av, Tv), or manage both (Manual). Of course, on Manual you had to take care of the exposure.

Now all three variables are available to us, so now we can let the camera do all three (Program/Auto mode and Auto ISO), or any combination of fixing one, two or all three of the variables.

Each variable has it's own characteristics (like the ones for ISO that have been well described in this thread), so it's really just a case of deciding which ones matter most for the image that I am trying to make (and which compromises I'm prepared to make), and letting the camera take care of the rest - or telling me if it can't!. Or controlling all three, of course.

I'm sure the details vary from camera to camera, but that's my idiot's guide to me.

Your statements perpetuate a common belief that is completely wrong. When using Av, Tv, or P mode, the camera isn't deciding anything for you. A much better way to put it is that the camera is acting on your behalf...but the camera is doing exactly what you're directing it to do. You have just as much control over exposure in Av, Tv, and P modes as you do in manual mode.

Cameras had "Av" and "Tv" modes long before there were any electronics in the camera. They were wrapped up into one system called the EV Interlock. The meter, if the camera had one, was usually an independently functioning piece of equipment...it was part of the camera but it didn't interact with the camera. Instead, it would give a reading in Exposure Value. You would then transfer that EV to the lens by hand. The lens would be marked in aperture, shutter, and EV. Once the EV was set, you'd engage the EV Interlock. Now, selecting your desired aperture would automatically change the shutter (Av), or selecting your desired shutter would automatically change the aperture (Tv.) You would over or under expose the scene by selecting a different EV than what the meter recommended (just like Exposure Compensation on a modern camera.)

What's important to realize is that there is no difference in the selection of aperture/shutter between that old EV Interlock camera and a modern camera. The camera isn't making any decisions for you, it's simply performing an adjustment according to a set of rules...an adjustment that you would have made yourself in exactly the same way if you were in full manual mode. This is the only automation that is provided by the auto modes of Av, Tv, and P.

P mode provides an additional function of selecting a particular aperture/shutter combination for a given EV. Again, the camera is simply following a set of rules...in this case, a program chart that is usually listed in the appendix of the camera manual. You can look up an EV, and find the aperture/shutter that will be set for that EV (which sometimes also varies with focal length, and that will be in the chart as well.) Most cameras also allow the program to be adjusted on the fly if you desire a different combination.

It doesn't matter whether it's M, Av, Tv, or P mode...the photographer has complete and precise control over all exposure parameters. If he feels he doesn't, then what's needed is not M mode, but a lesson on how his camera works.

[davidedric]

If he feels he doesn't, then what's needed is not M mode, but a lesson on how his camera works.

It may well be true that I need a lesson in how a camera works

But I don't think this is it

Yes, I know I can have complete control: the first point I was trying to make what that there is now an extra variable, ISO, that I can choose to control, and which I couldn't on a film camera.

When I say that the camera "decides" I meant, for example, that if I have chosen not set a shutter speed the camera's circuits may, depending on model and mode, set a shutter speed that tries to avoid camera shake. A guide for the unwary - no more than that.

Why might I not want to exercise total control? I'll give an example. I shoot a lot of wildlife. I may expect any action will be happening very quickly and that I want to freeze it as much as possible - so I will set an appropriately high shutter speed. I may or may not decide that I want to set a particular aperture. However, I do not know where the action will take place, and so I cannot predict the lighting conditions. I will then concentrate on framing the shot, and let the camera "decide" on an appropriate ISO.

It helps me to have a simple model in my head. I'm sure that there are heads larger and more experienced than mine that don't need such.

Enjoy and learn - that's what we are all doing, I hope.

[Graystar]

Yes, I know I can have complete control: the first point I was trying to make what that there is now an extra variable, ISO, that I can choose to control, and which I couldn't on a film camera.

When I say that the camera "decides" I meant, for example, that if I have chosen not set a shutter speed the camera's circuits may, depending on model and mode, set a shutter speed that tries to avoid camera shake. A guide for the unwary - no more than that.

Okay. Yes, that's how the Scene modes work....the Sports Scene mode, for example, will select both a fast shutter speed and high ISO in order to freeze action. With those modes, the decisions are completely out of your hands. But I don't view Av, Tv or P modes that way.

Why might I not want to exercise total control? I'll give an example. I shoot a lot of wildlife. I may expect any action will be happening very quickly and that I want to freeze it as much as possible - so I will set an appropriately high shutter speed. I may or may not decide that I want to set a particular aperture. However, I do not know where the action will take place, and so I cannot predict the lighting conditions. I will then concentrate on framing the shot, and let the camera "decide" on an appropriate ISO.

In that scenario I would still consider myself to have total control. I shoot with a Nikon DSLR. By configuring a few settings beforehand, I can basically instruct the camera to behave a certain way. When I go to shoot birds I'll set my AutoISO function. In the Nikon AutoISO, a Minimum Shutter Speed is set. The ISO will remain as low as possible until exposure conditions would slow the shutter speed below the MSS. In that case, the MSS would hold, and ISO will begin to increase. In that way, a minimum shutter speed is maintained and the ISO is always as low as possible. This allows me to shoot in aperture mode while knowing that I'm protected against too slow a shutter speed. The important thing to realize here is that even though the camera is controlling shutter and ISO, it's doing so as I have directed it. I will get the same exact shutter/aperture/ISO combinations that I would have set myself were I shooting in manual mode. For that reason I consider myself to be in total control of the exposure.

This is the value that modern cameras provide...functions that provide increased, real-time control over situations where, in the past, you'd have taken a good guess at the settings and hope for the best while shooting.

It helps me to have a simple model in my head. I'm sure that there are heads larger and more experienced than mine that don't need such.

I have a big head...or so I've been told...many times Yeah, simple is nice. On my old Canon F1 I'd worry about aperture and shutter. Today, it's aperture, shutter, ISO, white balance, metering mode, drive mode, AF mode, AE-L button setting, FUNC button setting, battery level...I have to run through a checklist every time I pick up my camera! But I feel that there are worthwhile benefits from the added complexity.

[davidedric]

I think we really have a heated agreement. Take care. Dave

[Donald]

What we strive to encourage on CiC is a constructive exchange of views and opinions, not heated arguments. I hope everyone will proceed on that basis.

[Manfred M]

Yes, I know I can have complete control: the first point I was trying to make what that there is now an extra variable, ISO, that I can choose to control, and which I couldn't on a film camera.

Fortunately, or perhaps unfortunately, I count many more "variables" that are now offered the the digital camera versus what film cameras gave us over and above the three that you list.

White balance was easy with film; I could buy daylight or tungsten film; I wasn't something the camera decided for me if I let it. I could introduce grain as a compositional element by choosing a faster film and pushing it in the darkroom (somehow, digital noise does not seem to be a very good replacement for film grain). Then of course there are the "biggies" like auto focus. I still remember razor sharp focusing screens that just "popped" when an image came into focus. Image stabilization meant shooting at a shutter speed where I could hand-hold the shot, without relying on technology to help me out. High speed shooting mode was totally dependent on how fast I could run the film-wind lever. I could go on and on.

Let's face it. We are no longer shooting cameras in the traditional sense, rather we are using computers that take pictures. Algorithms and technology drive the speed, focus, aperture, white balance settings, and too many photographers assume that the engineers and software designers must know best. Even more unfortunately, too many photographers leavethe camera, permanently set on automatic mode, and thereby make pretty well every decision for them except perhaps how the image is framed.

I pretty well always manually select the ISO setting and shoot either aperture priority or shutter priority mode, choosing either the aperture I want or the shutter speed I want. I will even resort to manual settings to tweak things that I don't trust the camera to do right. Used well, modern technology gives a photographer that understands the variables a lot more control that he or she had in the film days. Unfortunately, there are too many photographers that don't understand how the camera works and throw their trust to the camera's design team. I do pity them, as they don't understand what they are missing. After all they spent a small fortune on the camera and accessories, so they must be getting great shots?

[davidedric]

Thanks, Manfred, excellent post.

Let's face it. We are no longer shooting cameras in the traditional sense, rather we are using computers that take pictures.

That's the core of it. Our camera's now are awesome, it's what someone called the IBM (Idiot Behind the Machine - love that) that hasn't changed.

I never had a Darkroom (well, except for "hard" science research photography, which is a whole different game) and on top of the camera I can now do things in Lightroom that I never dreamed of.

And even better, we can talk across the planet about it. I'm off to bed, and I think it's great.

[NikonFL]

Hello
I'm still a beginner in all this photography business, and am getting my head round the three components of exposure - shutter speed, aperture and iso. The first two are straight-forward, but I can't work out why a digital sensor needs iso. As it is a digital device, why can't the sensitivity always remain at the highest level for the particular sensor? Why should you need to adjust a digital sensor's sensitivity?
Anyone know a simple answer to this?
Thanks

Hello Clive,

I've been reading responses and searching the internet. Would you believe WIKIPEDIA offers the simplest answer???

http://en.wikipedia.org/wiki/Film_speed

Stated in Wikipedia, "Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to measure the sensitivity of digital imaging systems."

Text is precise, thorough and appears to have been developed by industry experts.

[cliveg]

Hello NikonFl,

Thanks for this, I know what you mean about Wiki being a definitive source, shocking!

I think we have arrived at the best of all worlds here - a simple answer for simple minds like mine, and all the technical stuff for those that want to get into the minute detail.

I think we will gloss over the arguements, and instead (as David from Cheshire says) celebrate the fact that we can bounce ideas around the world!

[NikonFL]

Hello NikonFl,

Thanks for this, I know what you mean about Wiki being a definitive source, shocking!

I think we have arrived at the best of all worlds here - a simple answer for simple minds like mine, and all the technical stuff for those that want to get into the minute detail.

I think we will gloss over the arguements, and instead (as David from Cheshire says) celebrate the fact that we can bounce ideas around the world!

Clive, Thanks to your terrific thread, I reviewed the Exif data on a couple of photos I had taken at a Cirque du Soleil performance 1where cameras are not allowed. On one the ISO was 2500 and on the other it was 3200. As long as your NOT shooting in Manual Focus, the sensor knows exactly what "ISO" is needed.

I have always ignored choosing ISO in digital. Playing around, I set the ISO to 6400 and let the camera choose the aperture and shutter speed. Picture looked ok until I enlarged it. Awful grain...

[Colin Southern]

As long as your NOT shooting in Manual Focus, the sensor knows exactly what "ISO" is needed.

I think you might mean "manual exposure", not "manual focus" and "camera" not "sensor"

Playing around, I set the ISO to 6400 and let the camera choose the aperture and shutter speed. Picture looked ok until I enlarged it. Awful grain...

Noise absolutely will increase with increasing ISO as you're effectively using less and less of the sensors capacity, and thus operating closer and closer to the noise floor (think of it as a water well where there's clean water at the top and muddy water at the bottom ... increasing the ISO is like lowering the water level of the well (or using less and less of the well's capability) ... the more you lower it, the closer you get to the muddy water). The thing to remember though is that noise is very small - far far far far too small for the eye to see when looking at the entire image on a screen or when looking at a "normal" size print; or to put that another way, about the only time the noise is visible is when looking at the pixels at high magnification - so - why do it? If you want to check how clean your carpet is, do you stand at the door and look, or do you get down on your hands and knees and go over every square millimetre with a microscope?

[Donald]

If you want to check how clean your carpet is, do you stand at the door and look, or do you get down on your hands and knees and go over every square millimetre with a microscope?

No, my partner does that and demands to know why I spend so much going on about photography on a forum when I should be cleaning the house!

[William W]

Simple answer:

If Digital Cameras did NOT have different ISO settings, (or that control by another name) then we would be carrying a lot of ND filters and a lot more Lights.

And even then, with all those filters and lights we would NOT be able to make the range of images as simply as we can with an ISO control.

WW

[Colin Southern]

No, my partner does that and demands to know why I spend so much going on about photography on a forum when I should be cleaning the house!

Humph ... tell her cleaning is "woman's work"! (and then run ... run like the wind ... run like you life depended on it!!!)