need some help understanding ISO

could someone please help me understand what exactly ISO is, as of right now all i know is that the better your ISO the better you can shoot in low light

ISO is the film (media) sensitivity to light. The lower the ISO the less grain you get in film, but the slower your shutter speed will be. The higher the ISO, the faster your shutter speed can be, but the more grain you get.

Now that being said, in digital format, ISO is still how sensitive the sensor (media) will be. Just as with film, the shutter speed reacts to the same settings. Just replace grain from film with noise for digital.

Check out a book by Bryan Peterson, Understanding Exposure. It will teach you to understand the exposure triangle. ISO, Shutter Speed, and Aperture (f/stop). It is the best book for understanding the relation of one setting to another, and how to compensate for a change in any one of those.

Thank you, PavementPilot, that was very insightful!

You're welcome. Glad I was able to help you.

Yup, very nicely explained! You can also check out this guide to understanding ISO from Nikon. Because it's Nikon specific, it can help you navigate the ISO-related settings in your menus.

To take PavementPilot's explanation, which is correct, a little further, ISO stands for International Standards Organization. Which is incorrect, the organization that regulates digital (film) sensor sensitivity is named International Organization for Standards. Before ISO came along in the US it was regulated by the American Standards Association, or ASA. The base standard for ISO was initially established at 100. This number was used as the optimal return for grain size. Below 100 the smaller grain size that is gained becomes substantially less. Above 100 the grain increases starts to become substantially greater. (This base standard was developed for the now very old film technology.) In more recent years the fast developing digital technology has stood this standard on it's head. The development of digital sensor quality without the subsequent increases in grain size has about doubled the old standard to 200 with 400 fast on it's way to giving a grain size equal to the old standard of 100. Very substantial progress has also been made in the reduction in grain at even higher levels so that shots taken at ISO 1000 or even 2000 does not produce the very objectionable grain that it use to. There has however been something of a backlash with the development of less grain and many B&W photographers consider grain as a part of the artistic value of a B&W shot. So that there are any number of programs on the market that will emulate grain. They even go so far as too emulate the grain of specific films and film manufactures.

Joseph Bautsch said:

...There has however been something of a backlash with the development of less grain and many B&W photographers consider grain as a part of the artistic value of a B&W shot. So that there are any number of programs on the market that will emulate grain. They even go so far as too emulate the grain of specific films and film manufactures.

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At the risk of derailing this thread, I completely agree here. Current digital technology has created a need for a new standard, as digital noise levels are so low at current ISO standards, they might as well not even have a standard anymore. As I shoot a lot of B&W, I've had to purchase a set of presets for Lightroom that will simulate film grain, because it's nearly impossible to achieve the look I want straight out of the camera.

Anthony Hereld said:

At the risk of derailing this thread, I completely agree here. Current digital technology has created a need for a new standard, as digital noise levels are so low at current ISO standards, they might as well not even have a standard anymore. As I shoot a lot of B&W, I've had to purchase a set of presets for Lightroom that will simulate film grain, because it's nearly impossible to achieve the look I want straight out of the camera.

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That's funny - I actually noticed the grain on some of the converted black and white photos from my wedding and didn't realize it was on purpose. It makes sense as to why the photographer did that. She uses lightroom as well.

Hi guys, I read all the advise to need some help re ISO, and it cleared up so many issues for me re print film, but what of slide film?? I only used slide film for a short period of time but still have some 2000 color slides. I always thought they were better than prints because as I was told that prints are negatives where as slides are positives & therefore don't have the same problems with grain, I thought they were right because I almost never had a problem when shooting slide film. Even B/W slides came out perfect. I used to use Kodachrome 25 / 64 and Echtachrome I can't recall the asa nbr right now. So I guess I need to know if slide film was also subject to the same grain problems???

Kodachrome and Echtachrome came in ASA speeds of 25, 64, and 100. Yes color film was subject to grain problems just like B&W. The grain problem with B&W prints is more prominent because of the enlargements made from the 35mm film to what ever size print being made. The larger the print the larger the grain. With color slides being projected on a screen the grain was not noticeable. However if you are printing color negatives to paper the grain would be noticeable just as with B&W depending on the enlargement size. With 35mm B&W and color negatives I could make 8"x10" prints that were relatively fine grained. Going up in print size from there the grain started to show up.

Hi Joe, thanks for your reply, I have to tell you that I never noticed grainy color slides when viewing them in the past, I'll have to look at them again to see if there was any & try to compare them to my digital photos if thats even possible. I don't know if my slides are still even viewable, as I haven't touched them in a dozen years & only to store them when I did touch them last.
But I want to thank you & the rest of the people whose advise was well taken.

Joe Cottone