What Does ISO Mean in Digital Photography?

T.C.

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I have a question about photo theory. I'm not a photographer, but I have casual contact with digital cameras, and I've always been confused about what ISO means in the digital context.

My question is this: When I change the ISO, am I changing the information the camera collects, or am I just changing the way it processes information?

When I change the aperture setting, I'm clearly changing the information the camera collects -- I'm changing the pattern of light that hits the receptors. When I change the shutter speed, I'm again changing the information the camera collects -- I'm giving the receptors a different amount of time to aggregate the light that hits them.

When I change the ISO, however, I don't seem to be changing the information the camera collects in a real way; I seem to be changing only the way the camera processes the information reported by the receptors. Am I right about that, or am I missing something?


-TC
 
Welcome to TPF

Here is a page that will have great links for you to search and read. Google the same terms, and you will find endless reading that will also help, including other sites.

Digital Photography Tutorials
 
pbelarge,

Thank you for the reply. I appreciate those tutorials. However, I'm not sure my question is answered there.

For instance, one of your tutorials claims that ISO speed "controls the sensitivity of your camera's sensor to a given amount of light". Is that really true, or do the sensors in digital cameras have fixed sensitivity, and the ISO speed is simulated by processing that happens after the sensors have done their job? That is the essence of my question.


-TC
 
Think of it like you are increasing the amplitude of the sensor and its ability to gather light. It is just like cranking up the volume on your TV, you get louder sound, but it will likely be less clean than at a lower volume.

The higher Iso you have the better the camera will perform in low light situations, but it also increases the image noise.

Noise it the random fluctuations of light and color that gives it that grainy/dirty look.

Check out this guide that I wrote if you want to know more about sensors and how well the different types perform. There is info on Iso in it as well:

http://www.thephotoforum.com/forum/...-photo-gallery/219125-dslr-sensors-guide.html

Hope this helps.

Neil
 
ISO (International Organization for Standards). In terms of photography, they use the same 'scale' as film for simplicity.

In digital photography, the camera has a native ISO. That is basically the setting at which the camera collects light. In most DSLR cameras, I believe that it's ISO 100, although several Nikon models use ISO 200.

When you turn up the ISO, the camera amplifies the signal from the sensor. So nothing really changes at the sensor level, but somewhere between the sensor and the image being written to the card, the signal is amplified/boosted/gained etc.

The effect is similar to turning up the volume on your stereo. You get more output from the input...but you also get more interference. On your stereo, you may get more static or crackling etc, but in digital photography, you get more 'digital noise'.

That's how I understand it anyway.
 
Neil,

Thank you for the reply.

You seem to be answering my question when you say that changing ISO is "increasing the amplitude of the sensor and its ability to gather light". That would imply the sensors in a camera have a range of operating modes that can be changed by the ISO setting.

However, your DSLR sensors guide says "The sensor basically just records the light coming through the lens, and converts it into electrical signals that the camera then processes into image data." That statement implies the opposite -- that the sensors don't implement ISO, but the camera's processing firmware does.

Which is it?

-TC
 
So nothing really changes at the sensor level, but somewhere between the sensor and the image being written to the card, the signal is amplified/boosted/gained etc.

Thanks Mike. That's what I was trying to figure out.

So, here is my follow-up question: Is any information lost when the signal is amplified/boosted/gained?

-TC
 
Ya, as far as I know...it's the camera's processor that does the amplifying. At least, the camera companies brag about it when they update the processor in their cameras. (Canon Digic IV etc.)

So increasing the ISO doesn't necessarily increase the sensor's ability to gather light...it just allows the camera's ability to create an exposure with less light.

So, here is my follow-up question: Is any information lost when the signal is amplified/boosted/gained?
Hmmmm...I not sure. My first thought is to say, I don't think so. But I guess that someone could argue that when noise is introduced, it has to replace something (there are only so many pixels) so whatever the noise replaced, is essentially lost, or at least changed.
 
Neil,

Thank you for the reply.

You seem to be answering my question when you say that changing ISO is "increasing the amplitude of the sensor and its ability to gather light". That would imply the sensors in a camera have a range of operating modes that can be changed by the ISO setting.

However, your DSLR sensors guide says "The sensor basically just records the light coming through the lens, and converts it into electrical signals that the camera then processes into image data." That statement implies the opposite -- that the sensors don't implement ISO, but the camera's processing firmware does.

Which is it?

-TC

No what I said was not the opposite of what my guide says.

I said "Think of it like you are increasing the amplitude of the sensor and its ability to gather light"

How does this imply anything about the sensor other than that you are increasing its ability to gather light? I did not say that the sensor alone carries the entire process out, and it is you who is implying this. I just said it was increased, and did not say how exactly.

The sensor is simply that, a sensor. Like I said in the guide, it records light that the camera's electronics then process into image data.

It is the cameras Iso setting control, and its electronics, that controls the Iso sensitivity. The cameras electronics then "tell the sensor" how much to crank up the amplitude, and this then determines its ability to gather light.

Do I smell a troll disguised as a noob? You sure seem to know a lot for someone that doesn’t know much and needs help on this.

I basically said the exact same thing as Big Mike (before he did) and yet you are trying to say my post (or guide) is flawed while his post is not?

I am just trying to help here, and it is you that is asking for just that...
 
Mike,

If information is lost when ISO is applied, then I must wonder if professional cameras using the RAW format apply the ISO setting, or if they save the image in an unprocessed format and rely on post-processing to do the ISO transformation.

If no information is lost, then I suppose the question is moot.

-TC
 
Neil,

You have my heartfelt apology. You did indeed, give me a good answer, but I'm still wrapping my mind around this subject and I didn't quite understand. I appreciate your help and I'm grateful for your DSLR sensor guide. Thanks.

-TC
 
Mike,

If information is lost when ISO is applied, then I must wonder if professional cameras using the RAW format apply the ISO setting, or if they save the image in an unprocessed format and rely on post-processing to do the ISO transformation.

If no information is lost, then I suppose the question is moot.

-TC
Well, I don't think it's the same as saying that when you shoot JPEG, information is lost.
Even when shooting RAW, the data is affected by the amplification of higher ISO.
Otherwise yes, we would be able to adjust the ISO of a RAW file, like we can adjust the color temp of a RAW file.

In a technical sense, I don't think that any information is 'tossed out' as when you shoot JPEG only. My reasoning is that if some pixels in the image are replaced by noise...then that 'clean' information is lost.
 
When I change the ISO, however, I don't seem to be changing the information the camera collects in a real way; I seem to be changing only the way the camera processes the information reported by the receptors. Am I right about that, or am I missing something?
-TC

What’s important is that you understand that it changes the camera’s ability to gather light, and that noise is then increased as a result.

As a photographer this is pretty much all you need to know, along with the fact that larger sensors provide better Iso performance. By that I mean lower noise for a given Iso sensitivity.

I find it strange that you are asking such a detailed and in depth question, while appearing (at least) to not even fully know what it does.

How exactly the Iso sensitivity changes from when you press a button changing the setting, to the increased light+noise in the image on the card is pretty irrelevant for most people. That is unless you want to be a DSLR engineer or something. The specific software programming/electronics/electrical signals invloved would be way over everyones heads here, and we simply dont know the answers.

If you want to know more (beyond what we do) then I suggest you do research and read about it. The internet is a great place to expand your knowledge on just about anything.

This is why I said that I suspected you may be a troll in disguise fishing for a response. If this is not true then I apologize for the misunderstanding.

Neil
 
Mike,

I understand your explanation of ISO and noise, but I'm not sure it means applying ISO loses information. Here is my reasoning: I suspect high-ISO is not what makes a photo noisy, but low light. If the camera shoots a photo at its native ISO in low light, then amplifies the signal to achieve a higher ISO, the photo may not actually become any noisier in the process.

In any case, this avenue of discussion is leading us down a tangent. I've reached my original goal of understanding ISO better. Thanks.

-TC
 
Neil,

My questions are not motivated by a desire to become a better photographer, but by a desire to understand photo theory better out of pure scientific curiosity.

I should have mentioned that, although I don't know much about photography, I do know a little about human vision. Therefore, I approach photo theory with a clear awareness of how things could work differently. Aperture has a clear analog in human vision; shutter speed is less analogous, but still understandable in the context of human vision; but ISO speed is a foreign concept, and that's what gave rise to my questions.

In any case, you've been very helpful, and I feel like I learned what I desired to know. Thanks for your help.

-TC
 

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