What is ISO?

We know that exposure is controlled by the holy triumvirate of shutter speed, aperture, and ISO. You give with one and take with another until the light hitting your sensor is just right for the image that you want. We also know that whilst these guardians of our sensors have practical applications, they can be creative. Aperture has already been cross-examined in our Photography Fundamentals series; this week it's the turn of ISO.

Sensitivity to light

Once upon a time, in days of old, when we shot on film and had to fend off dragons simultaneously, we also had to choose the ISO of our film when we bought it. 100? 200? 400? Huh? Now it's just a button on our cameras and we can change it every shot. But that doesn't mean that ISO is any less significant now than it was then. In fact, changes to ISO have been one of the major breakthroughs in modern photographic technology.

Back in those old film days, ISO referred to how sensitive a film was to light. Depending on the conditions that you thought you'd be shooting in primarily, you'd have to select the ISO of your film and be done with. (If you want to really geek out on ISO trivia, Haje has you covered.) Now you can push a button and see it increase from roughly 100 to anything as high as 102,400 (but that's extreme). That's a lot of numbers but they still mean the same thing: it's how sensitive your sensor is to light. (Or, technically, how much the signal from each pixel on your sensor is amplified.)

Flash in an aquarium? Bad plan. ISO 800? Gets the shot!

The higher the number, the greater the sensitivity to light.

Low vs High

The general rule is that the brighter the conditions, the lower you want your ISO to be. As conditions become dimmer, you can increase sensitivity to help you get a good exposure. However, even if it's a low-light situation, you still want your ISO to be as low as possible.

There's a trade-off, you see. As you increase sensitivity, you also increase the tendency towards digital noise, or the graininess you see in some photos. Now we do have a full Photography Fundamental on noise lined up for you but the low-down is that too much noise can make images look poor. It's best to avoid too much of it.

Whilst too much noise does give me a headache, I'm more than prepared to sacrifice a bit of image quality in order to secure my shot. Yes, I'll take a smidge of noise over motion blur almost any day. (Unless I really want motion blur, that is.)

Yes, it is a bit noisy, but I'd rather the noise than no photo

Which ISO to use

Well, seeing as ISO forms part of the mighty triumvirate with aperture and shutter speed, you'll need to consider those in order to get the exposure that you desire. But do stick with the rule I mentioned earlier: as low as you can get away with.

Equivalent exposures

It's really easy to leave your camera to automatically select the ISO for your shots, but it is a really useful tool. And the more control that you take over your own images, the better. Don't just concentrate on aperture and shutter speed.


  • ISO refers to the sensitivity to light of film or the digital camera's sensor.
  • The higher the number, the greater the sensitivity.
  • A low ISO generally offers the best image quality, and great for brightly lit situations.
  • A high ISO setting may be required to capture images in low light situations, but can cause noise to show up in your photos

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