What is ISO?

There are three settings on your camera which control exposure they are ISO, shutter speed, and aperture. Together they are frequently called the Exposure Triangle. ISO refers to the International Organization for Standardization (I know that should be IOS). I've looked and I have not yet found a language in which the name would come out ISO but it is what it is and we've dealt with it since 1947.

In photography it refers to sensitivity to light. In film days we bought 100 speed film, those of us who shot slides sometimes shot 25 speed film. If we were going to shoot inside in dim light we bought 400 speed film. In those days the film was usually labeled ASA (the American Standards Organization) and at a time I've lost to memory the boxes began saying ISO instead of ASA. The numbers didn't change. ASA 100 and ISO 100 are the same speed. ISO and Speed measure how sensitive the film is to light. The higher the number the more sensitive the film.

Remembering "What is a Stop?" ISO's double to make them one stop more sensitive. If you are shooting at ISO 100 and you change it to ISO 200 (this only works in Manual mode) your picture will be one stop brighter; change to ISO 400 and it will be 2 stops brighter. In automatic modes changing the ISO causes the camera to automatically adjust the shutter speed, aperture or both (these being the other two sides of the Exposure Triangle) and the image stays the same exposure (brightness).


In digital photography the numbers still have the same meaning only now it refers to how sensitive the digital sensor is to light not the film. In film days if we wanted more sensitivity we removed the film from the camera and put in a more sensitive film. In digital cameras we can change the ISO for each shot if we want to, because there is no film to change.

To truly change the sensitivity of the digital sensor we would have to replace it with a more sensitive one. Every sensor has a "native" sensitivity that cannot be changed. So why do we think we can change it? What comes out of the sensor is an electrical signal and electrical signals can be amplified. If we are listening to music and want it louder we "turn up the volume". If the picture is not bright enough we "turn up the volume". In dim light the signal coming from the sensor is weak so we amplify it which unlike sound doesn't make it louder, it makes it brighter. We have "effectively" increased the ISO.

The next question is: "How much can we do that?" and that answer changes as improvements are made in newer camera models and also with the price of the camera. Here is the problem; every electrical circuit contains two types of information, "signal" which is what we put there and want there, and "noise" which is what we don't put there and really don't want there but it's a physics problem, where there is signal, there is also noise. In digital photography the signal is the picture and noise shows up, mostly in dark shadow areas, as colored flecks or spots. When we amplify the signal, the picture gets brighter. But we can't just amplify the signal, we also amplify the noise and it gets worse. we get more colored flecks until it gets to the point where the image is worse than we are willing to allow, and then we have passed the "How much can we do that?" point.

So where can we expect to find the point. Most DSLRs on the market have a "native" ISO of 100, some start at 200, also most have little problem getting to 1600 ISO without excessive noise. There are quite a few cameras that will go higher than this and the highest speeds are frequently called extended or expanded range which means your images no longer appear "noise free". The Nikon D4 as an example will hit 12,800 ISO without using the expanded range, but it is a $6000 camera and for that money the expanded range will take you to 204,800.

The series (above right) is the section outlined in yellow (left) of photos that were taken with a Nikon D200 and from the top down were shot at ISO 100, 200, 400, 800, 1600, 3200 each is one stop faster than the previous. The 3200 was an extended range ISO.

What does this all mean in practical terms? The photos were taken on a cloudy day inside and the room was not very bright. At ISO 100 the shutter speed was 1/5 second and probably could not have been hand held. In fact until the ISO was all the way to 3200 (that's 5 stops faster) the shutter speed was too slow for someone my age to attempt without a tripod. The bottom line is that higher ISOs allow faster shutter speeds and save you from setting up a tripod for many shots.