What the ISO?

What is ISO speed? Put simply, the ISO speed¹ is a measure of how sensitive our film or detector is to light. I'm going to ignore film here for the most part, but most of what I say about detectors also applies to film. ISO is expressed as a number. The scale is linear, meaning that ISO-100 is twice as sensitive as ISO-50, and half as sensitive as ISO-200. This sensitivity has two distinct effects.

The more sensitive our detector, the more each photon of light hitting it affects our resulting image. With our detector set at very high sensitivity, we need less light for the same resulting brightness.

From this animation, you can see that the brightness of the image increases as our ISO increases, as expected.

Awesome, right? If we just boost the sensitivity of our detector, we can take pictures with less light.

There's a drawback though, which leads us to the second effect of ISO speed. You may have noticed that the brighter images have some significant noise. Here's the bottom right corner of the ISO 12800 and 25600 images. If you look at the darkest parts of the image, you can see the noise, which appears as speckle. It should also be fairly clear that the higher speed image has more noise than the lower.

As with everything else in photography, we have to make trade-offs. What part of our image are we willing to compromise for the sake of another? Are we willing to accept some noise for an image that's well exposed, or do we want an underexposed image with less noise?

There are two other significant parameters we can adjust for shooting at night. We'll look at them in the coming weeks.

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¹ You may hear some old timers (myself included) refer to the ISO speed as ASA speed. The two numbers are functionally identical. I'm going to completely ignore the DIN side of the ISO spec for the sake of simplicity.