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.

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

It seems my short answer is being misconstrued as a rant or just a BS remark.
So I guess, a long answer is needed.

First of all, the comment is my own stand and I am not pushing my agenda on anyone. Just clarifying my view and from where I am looking at it.

I meant no BS for anyone and literally meant what is written there. IN THE DIGITAL WORLD - that comment about ISO is only about Digital cameras & how the manufacturers distort it for their gain (pun intended).

Ysarex made a very nice post about it and I am not contesting that. I did however follow up an answer that (for me) it does not matter what those ISO standards are in relation to cameras because it allows the manufacturer to make their own standards. Hence the reality is that there is variance or to make it more blunt, the ISO from one camera model and one manufacturer to another are not the same.

Hence I say, just use it as a tool like a volume control but something that affects brightness instead of loudness. This is meant to relate one idea which is common to many, to an unknown. It was not a literal conversion, just the gist of the matter.

The above reasons are why I do not bother with ISO standards as it is not standard amongst the tools it was supposed to be on. I take it at face value and use it like what it used to be in the film days but knowing fully well it is a different technology.

If anyone wants to be really technical and know the full details read up on an this previous post:
https://www.uglyhedgehog.com/t-671081-1.html

https://www.visiononline.org/userassets/aiauploads/file/cvp_the-fundamentals-of-camera-and-image-sensor-technology_jon-chouinard.pdf
Take note on pages 52-59

To really appreciate and compare one camera with another, view this video:
https://www.youtube.com/watch?v=mqG7Raj-iWo

The above are engineering & manufacturing information and are the real essence of the equipment.

For those who wonder why there is a variance in ISO, and why manufacturers are allowed to make their own standards, consider the following:
1. Technology changes rapidly
2. Manufacturers do not like to share technology
3. Digital ISO is trying to put a definite value on the outcome of different physical equipment matched with different processes.
I.E. CCD vs CMOS & each manufacturers has their own Analog-digital processes.
4. Age. As equipment age, its performance will also change.
5. Advertisement & Sales
6. Business vs compliance
7. Digital ISO is a product of many parts & process within the camera, not a single sensitivity value that ASA is.
8. Digital ISO is meant as an equivalent or transfer of previous knowledge, so it was made for representing something else it was not.

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.

I really do not want to answer your post as we never look at the same thing with the same understanding but you are forcing me to do so.

These are excerpts from you previous posts supporting my claim:

"ISO 2721 uses the term nominal exposure and assumes that the nominal exposure is an arithmetic mean exposure value, which usually corresponds to the mid-tone in photographs of average scenes."
(Nominal, assumes & usually are words that I would not use to claim a standard.)

"Start with Gene's post a couple pages back: https://www.uglyhedgehog.com/t-673105-2.html#11729175 and read that excerpt and note that it refers multiple times to two different measures for ISO: SOS (Standard Output Sensitivity) and REI (Recommended Exposure Index) ."

"In practice however the distinction between those values amounts to about 1/2 a stop.
The REI measure contains a fudge factor. The ISO standard is written to allow each camera manufacturer to use either measure with the caveat that they have to tell you -- it's in the exif data."

"The end result is that if you own a camera that uses SOS to determine ISO values and another camera that uses REI to determine ISO values they won't meter the same exposure identically at the same ISO values. My Fuji cameras use SOS and my Canon uses REI."