What the heck is ISO anyway? A sensitive question.

Setting ISO is often described as setting the sensitivity of the sensor. This…works. Kind of. It’s not technically true, but the value of the practically useful explanation holds up for a long time. For most practical purposes, it just doesn’t matter.

This is one of those recurring topics on forums where people snipe at each other endlessly without convincing anyone of anything. In fact, one such thread prompted this, and it started as a “photography myths” thread which is now entirely a few people going back and forth. It is tiresome, and they’re mostly talking past each other based on some misreading a page or ten back.

Guess why I stay away from forums these days.

So here’s the facts. No meanness. No smugness. Just a basic explanation.

Sensors are electronic things, and electronic things have electronic noise. Camera sensors are covered in little techo-bits called photosites. Each one measures a tiny little amount of whatever photons land on the sensor, usually after it’s passed through various filters that cut out certain kinds of light like infrared and ultraviolet. As the camera samples more of that signal coming off the sensor, more of the noise is tangible. If you’ve ever played with the gain on your laptop microphone and heard noise as it goes up, it’s a similar thing.

As the noise increases, our subjective ability to tolerate its impact on the aesthetic quality of the signal declines. Higher quality sensors and signal processing pathways between the sensor and the media it’s recorded on reduce the noise, and so reduce the aesthetic impact of increasing the gain.

A photosite on a digital camera sensor always measures the same amount of noise along with the signal of the photons hitting it. Setting the ISO sets the gain, though it’s not like a gain knob on audio equipment. The ISO we talk about with cameras is a standard produced by the International Organization for Standardization that defines the relation between gain and the ISO number as it relates to the shutter speed and aperture. So while it’s technically true that ISO controls sensitivity, it refers to the sensitivity of the camera’s signal processing pathway, not the sensor itself.

The sensor always has a certain amount of noise, fixed on the day it was produced, based on choices made regarding target market and available chip-making technology. Better sensors have a lower noise floor and thus reveal less noise as the camera samples at ever-higher ISO levels. Cheaper, smaller sensors have more, so turning the camera’s sensitivity up via ISO will reveal more of the ever-present noise while also showing more of the recorded photons. This tends to hit a subjective point where the electronic noise overwhelms the aesthetic quality of the recorded light. We’re likely a few sensor generations from noise being a non-issue at all practical ISO settings. Expect a renaissance in night and astrophotography.

Generally speaking, the better you expose a photo via shutter and aperture, the more it drowns out the noise. This is why dark portions of photos often seem noisier. Less photons, higher percentage of the ambient noise at the same sampling rate. Most cameras have an option to sample a shutter-closed frame equal to the length of the exposure to subtract the noise pattern from the image since this usually involves scenes with very little light.