eli5 why there is noise in pictures taken in the dark?

Question

I was always wondering about this because I don’t understand why I can see random colorful pixels when shooting something in the dark…why is that?

In: 3

Answer 1

The challenge with taking a photo in the dark is obviously that there’s less light for the camera to work with. One way to go about adjusting for this is to make the film/sensor more sensitive to light. In film cameras this was done by actually using a different film with a different chemical composition that was more sensitive, this sensitivity was measured in ISO. With the introduction of digital photography this ISO measurement was carried over, but instead of measuring film sensitivity it now controls gain on the signal coming from the sensor. Kinda like a microphone, turning up the gain boosts the signal coming in making it louder, but too much gain and a loud noise will get all distorted. The same happens for the camera sensor, high ISO on a sunny day can over-expose the image.

The reason for noise in a dark image is because of a quirk of the high ISO. There is essentially less information for the camera to work with in a dark photo, think of it like this: Say each pixel on the camera sensor can tell you how much light hit it on a scale from 0-10. In a dark picture maybe each pixel only measured a 0, 1, or 2. Turning up the ISO is kinda like multiplying each of those values by 3 in the final image. The problem with that is the only values we can get out now are 0, 3, and 6. In a picture taken on a bright day they’ll be a nice gradient between any light and dark spots ranging from 0 to 6. But our boosted image has all these artifacts of sharp brightness changes, and we see noise in our picture.

One addition: the way around this is to increase the length of exposure. A long exposure lets more light reach the sensor, and allows the ISO to be kept low enough to avoid noise.

Edit: another thing. Film cameras also had “noise” at higher ISO but it causes the film to look grainy, not noisy. This is because the light causes actual grains of silver to crystallize on the film when it hits it. The higher ISO film stock works with less light by growing bigger grains of crystals, so it looks grainier.