three.toed Loc: South East Arizona

Hello - New to this forum. I used to take a few pictures with slides and I maybe understood what ISO meant (the amount of chemical on the film that provided more or less light to be captured and hence more or less detail). I cannot figure out what ISO does in Digital Cameras. This image of two Norther Mockingbirds Squabbling was taken at F4 - 1/2000 - ISO 100. Where is the sweet spot for ISO or is that all a matter of personal choice? I understand a little about speed and ISO but what does ISO really do? Thanks for any information or comments you can provide.

Ysarex Loc: St. Louis

ISO in the digital world is not the same as film ISO. With digital cameras ISO establishes a standard methodology for determining the lightness in the camera output JPEG that results from a measured exposure of the camera sensor.

When you increase the ISO value on a digital camera the camera metering system responds and calculates a new reduced exposure. That reduced exposure will darken the image unless an adjustment is made and so ISO then also lightens the output image by an equal amount to the exposure reduction -- it compensates. How that's accomplished can vary. The camera manufacturers use different methods (for different reasons) to get that job done. You can in fact think of that as a post processing job. The exposure is made and if the ISO value set on the camera requires it then the output image is lightened.

The most common method used by the camera manufacturers apply ISO lightening is to take the analog signal from the sensor and amplify it before the signal is converted to digital values. Another method that is increasingly common and that can be used in conjunction with the analog amplification of the sensor signal is to scale up the values after the signal has been converted to digital values. These two methods account for the way most cameras implement ISO lightening and it's critical to understand that the result is burned into the camera's raw file. These two methods are applied in the creation of the raw file.

ISO lightening can also be applied in the camera's image processor in which case the amount of lightening applied in the image processor is not applied to the raw file. You see this in some brands of cameras, Sigma for example, and to achieve specific results in some other cameras. A good example is the low ISO option (ISO below base) on some cameras.

The ISO sweet spot is base ISO. You get the maximum amount of DR from the sensor and the best signal noise ratio for the best overall image quality. But we can't always get away with using base ISO if we want to take low-light photos or fast action photos (your wildlife). In that case raise the ISO as high as needed to get the photo. Always get the photo.

Dynamic range and ISO are related. The two common methods of implementing ISO lightening noted above as they affect the raw file reduce the sensor DR. Here's a good reference for that: https://www.photonstophotos.net/Charts/PDR.htm

three.toed Loc: South East Arizona

I understand it is not, but what is it?

Ysarex Loc: St. Louis

I'm working on editing my first response -- keep posted.

johngault007 Loc: Florida Panhandle

Does ISO increase/decrease gain on the sensor? Someone please correct me, but increasing ISO amplifies the signal (light) to the sensor? One of the side effects of added gain for most transmission mediums, is an increase in signal noise. Which in the case of the camera adds a sort of "grain" effect.

Ysarex Loc: St. Louis

ISO can amplify the signal from the sensor. That is the most common way of implementing ISO lightening but it is not the only way to implement ISO lightening.


The noise most commonly associated with ISO is in the signal and not added by ISO lightening. It is revealed by ISO lightening because we're using a weaker signal to begin with. When we lighten a weak signal the noise that's there is made more visible. https://www.dpreview.com/articles/8189925268/what-s-that-noise-shedding-some-light-on-the-sources-of-noise

MrBob Loc: lookout Mtn. NE Alabama

Without going into the weeds can't we just say changing ISO lightens or darkens an image and base ISO for your camera will give you the greatest dynamic range and least amount of noise... This seems like one of those topics where the less said the better as it tends to confuse folks.

Ysarex Loc: St. Louis

Not really because if you use a camera in a semi-auto mode or if you zero the camera meter then changing ISO won't lighten or darken the image. ISO's job in that case is to maintain a constant lightness in the image by compensating for exposure changes.

Yes, ISO is confusing for a lot of folks. Unfortunately it's because they've typically been misinformed. If would take a lifetime to document all the Youtube misinformation out there about ISO.

IDguy Loc: Idaho

Many cameras use analog amplification up to a point and then above that point amplify digitally. The digital amplification is sometimes called “iso invariance”.

On many cameras the switch occurs above ISO 800.

For moving wildlife you want high shutter speed for sharp images; e.g. 1/3200. Although you can usually use minimum fstop for wildlife you might want to increase ISO to get that shutter speed. I use M, auto ISO, and spot metering to get what is needed.

johngault007 Loc: Florida Panhandle

I guess I shouldn't have put "light" in parenthesis in the context of talking about gain. ISO lightening is a new concept to me which I'll read about for sure.


So if increasing the gain of the signal and ISO lightening are two separate concepts, is ISO Lightening a result of some level of processing as opposed to increasing the signal strength?

johngault007 Loc: Florida Panhandle

Some of us like to dig deeper, and rely on the expertise of those that can explain it. If you don't wish to fall down that rabbit hole that's perfectly fine also.

Ysarex Loc: St. Louis

Amplifying the sensor analog signal is one way (most common) that ISO is implemented. Historically it was the overwhelmingly most common implementation method and unfortunately that led to some folks defining ISO by how it was done rather than by what it is. Since ISO can be implemented in other ways calling it something like "signal gain" becomes a problem. What is it when no signal amplification is used?

So up above I noted that what ISO does is establish a standard methodology for determining the lightness in the camera output image that results from a measured exposure of the sensor. ISO's job is to maintain that established level of lightness in the output image (the term ISO lightening). The ISO standard https://www.iso.org/obp/ui/#iso:std:iso:12232:ed-3:v1:en makes a point of leaving implementation (analog signal gain for example) up to the camera manufacturer and they can use any one of multiple options.

So if the ISO value is raised the camera meter calculates an exposure reduction. That will darken an image. ISO's processing job is to compensate for that exposure reduction. Implementation of that can be analog signal amplification and/or scaling of the digital values after the sensor signal has been digitized and/or simply lightening the image in the camera processing software. The camera manufacturer decides how they want to do it. Any of those methods can be applied separately or together.

Wallen Loc: Middle Earth

The short answer is:

ISO in digital world is a big lie perpetuated by the manufacturers to sweeten their sales.

cahale Loc: San Angelo, TX

To keep it simple; lower ISO, lower grain - higher ISO, higher grain. End result, modern digital cameras can take photographs at rather high ISO, in very dim light, and end up with, well, high grain and look like it.

Ysarex Loc: St. Louis

That is complete nonsense. Here's a link to the standard abstract: https://www.iso.org/obp/ui/#iso:std:iso:12232:ed-3:v1:en