Let's start by dispelling the myth: ISO does not stand for International Standards Organization. It's actually the adopted abbreviation for the International Organization for Standardization, an organization whose initials would be very different in different languages. That said, we use the term in photography because this organization was largely responsible for the development of standards for measuring the light sensitivity of film.
Now that we have that little bit of trivia out of the way, I'll get on with explaining what it means to you and how we use it.
What is ISO?
ISO, pronounced "I-S-O", "eye-so" or "ee-so" is still used describe sensitivity to light. In film cameras, it refers to the film's sensitivity and in DSLR cameras it refers to the sensor's sensitivity, which is adjustable. In this article, we'll deal with ISO as it applies to modern DSLRs. It's an important part of the Exposure Triangle, which we'll explain in the next article.
(Success Tip #1: 52 quick challenges to improve your photograpy in your spare time)
How does ISO work?
The ISO setting on a modern DSLR camera typically ranges from 100 or 200 up to 6400 or higher. The higher the ISO setting, the greater the sensor's sensitivity to light. Each higher setting is twice as sensitive as the one below it, so an adjustment of one setting makes a considerable difference.
It should be fairly obvious that higher ISO settings allow you to use faster shutter speeds and/or narrower aperture settings to achieve the same exposure. If not, reading the articles linked in the previous sentence should help.
How does ISO affect my images?
Aside from helping control exposure, ISO has one other main effect on your photos: it controls noise. As the sensitivity of the camera sensor increases, so does its tendency to produce digital noise. This shows up as grain and/or softness in your photos. For that reason, most shooting situations call for using the lowest practical ISO setting.
Shooting at the lowest ISO isn't always practical; otherwise there wouldn't be a need for an ISO adjustment. Low light, sports action, wind, camera movement and many other factors may require you to increase your ISO setting. In some cases, a photographer may want to use the noise created by shooting at a high ISO setting to crate an artistic effect.
How do I adjust my ISO setting?
The ISO adjustment is easily accessible on all modern DSLRs. As always, read the manual for your camera, but you should have no trouble finding it. If you find you're unable to change the setting, chances are you're using your camera in a preset mode. You'll have to take the plunge and switch to a mode that gives you more control.
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Are we done yet?
Yep! There's not much more to understand about ISO; it's one of the most simple concepts in photography. Understanding how it works with the other elements of the Exposure Triangle is a little more complex. As promised, that's the subject of our next tip.