A friend and I were talking today about film speeds in the days of film. 3200 ISO was about as fast a film as one could get, and it was black and white. There may have been colour film at that speed, though I doubt it.
So, a typical low-sensitivity film would have an ISO of 100. And a ‘fast’ film with more sensitivity would be something like 800 ISO.
What does ISO 100 and ISO 800 mean? It means that to get enough light onto the film to expose it sufficiently for the image to be produced, you need eight times more light on the slow ISO 100 film than you do on the fast ISO 800 film.
That’s because of the way photographers measure the amount of light. The standard way to measure ‘units’ of light is in ‘stops’. Each doubling of the ISO corresponds to twice as much sensitivity to light. Or one ‘stop’ of light as it is called.
Going from 100 ISO to 800 ISO means doubling once to 200 ISO, then doubling again to 400 and doubling again to 800 ISO. That is eight times more.
Of course there is a penalty for that increased sensitivity. The more sensitive the film the grainier the photo would be.
It all changed with digital cameras. No longer was the speed of the sensor in the camera fixed at a certain value. With digital cameras one could simply dial up a higher or lower speed, meaning a different sensitivity of the sensor.
Almost all digital cameras have a button or a dial to increase sensitivity, from the base sensitivity of 100 ISO (or perhaps 200 ISO) up to 400 or 800 or 1600, 3200 ISO and beyond.
And you may wonder how it is possible to just turn a dial and make a digital camera more sensitive to light.
And the answer is that just like with film, the more sensitivity the more noise.
No matter how big or how small the chip used in the camera, The more we boost the sensitivity of the chip by turning up the dial, the more ‘noise’ we get. And digital noise is the digital ugly sister of film grain.
That Was Then
Nowadays, though, most cameras are sufficiently good. There is hardly any such thing as a bad camera. They all focus quickly, make good photos, and make good photos at higher ISO. In fact, 3200 ISO is hardly ‘fast’ any more. 6400 or 12,800 is common.
This is the grab shot that I took to illustrate the point while Mark and I were talking about film sensitivity. It is at 3200 ISO from a Fuji X-E3. It is sufficiently good. It is so sufficiently good that it’s just completely acceptable. Noise? What noise?