Photo 101: ISO
August 1st, 2008ISO stands for International Organization of Standards. I don’t know why it’s not IOS, but what it refers to in photography is the level of light sensitivity of the sensor or film. It’s often called “film speed“, even when talking about digital cameras.
A higher ISO number means more light sensitivity; more sensitivity means less light is necessary to get a good exposure. Besides shooting a low light scene, a photographer might need to use a very fast shutter speed to freeze action, or a small aperture (large f/#) to get a very deep DOF. These situations may require getting the exposure with less light.
ISO 100 is typically used for bright, well lit scenes such as outdoors on a sunny day.
ISO 400 would be used in situations with less light, when photographing action, and/or with flash.
ISO 1600 would be used in very dim situations or with extremely fast moving subjects.
Generally higher ISO settings mean the photos will have more noise with digital or more grain with film. Whether noise is going to be an issue depends on how much magnification or enlargement the finished photograph requires. Noise won’t show as much in small prints or web friendly sized photos. On the other hand it becomes much more evident in large prints.
Currently available DSLRs have extremely low noise if the exposure is good. The difference between ISO 100 and ISO 400 is barely noticeable if at all, and even at ISO 1600 the noise is comparable to the grain of ISO 400 film. Underexposure is usually the culprit for excessive digital photo noise.
It also depends on the photographer’s opinion and vision of the finished photograph. For instance I think coarse noise/grain often works well with the subject of live music. Noise/grain is sometimes assumed to always be undesirable, but as my Photo 101 professor said “Grain is not evil. It’s just grain” (at least it was something like that). It is true that people do like sharp, smooth grained or noise free photos, but there are plenty of examples of great photos that have extremely coarse noise/grain. I think the detrimental effects of grain and noise on photographs are often over-emphasized in discussions among photographers. The rest of the world is mostly concerned with content.
Here’s a photo taken at ISO 100 or 200

Here’s one taken at ISO 3200 (no noise reduction used)

At these sizes it’s pretty hard to see the difference, but they would be obvious in an 8″x12″ print.
There are ways to decrease noise and grain. Film photographers can choose tabular grain films and fine grain developers. Digital photographers can use noise reduction software. With both there are exposure and processing techniques that can be used by the photographer to decrease or increase grain/noise.










