ISO: International Organization for Standardization.
In photography ISO or ISO number is a scale, a sequence of numbers used to describe the sensitivity of a digital sensor to light.
What is ISO in film photography?
In order to understand modern-day ISO numbers, we have to go back to the film age of photography.
Before the 1990s digital photography didn't exist. Photographers used photographic film, which is a strip of transparent plastic base coated on one side with an emulsion containing microscopically small light-sensitive silver halide crystals.The size of these crystals determined the sensitivity of the film to light. Bigger crystals made the film more sensitive. With bigger crystals, the photographer could shoot in lower light conditions.
In the film days of photography we didn't use the term ISO number but Film Speed. Film speed is the measure of a photographic film's sensitivity to light. This measure was expressed in several scales, depending mostly on the country where the film was manufactured. For example, Germany used the DIN scale. In the US we used the ASA scale. Eventually, the ASA and DIN film speed standards were combined into the ISO standards in 1974.
What does ISO stand for?
For digital photo cameras, the ISO setting is specified by the camera manufacturers such that the image files produced by the camera will have a lightness similar to what would be obtained with film of the same speed at the same exposure. In other words, an ISO 200 setting in a modern digital camera calibrates the sensitivity of the sensor in such a way that the image produced has the same lightness of a frame of film with film speed of ISO 200 taken at the same exposure. The Digital age of photography changed the system used to capture light but kept the same scale.
When we talk about ISO number or ISO setting in a digital camera, what scale, what sequence are we actually using?
ISO scale:
50 100 200 400 800 1600 3200 6400 12800 25600 and so on
This is the ISO scale that measures and describes the sensitivity of light of our sensor. It is an arithmetic scale where a doubling of the sensor's sensitivity is represented by a doubling of the numerical ISO value. Or the other way around. An ISO 1600 setting makes the sensor twice as sensitive to light than an ISO 800 setting. An ISO 100 setting makes the sensor half as sensitive than an ISO 200 setting.
We all know that photo cameras feed on light: photography needs as much light as possible to get a good exposure, to get pixel information. So why don't we always shoot with the highest ISO setting? Why don't we always shoot with the highest sensitivity to light that our camera sensor allows us?
Because unfortunately there is a trade-off between the sensor's sensitivity to light and image quality. The higher the sensor's sensitivity to light, the higher the ISO number the lower the image quality will be. High ISO settings introduce incremental quantities of digital noise.
This trade-off is unfortunate but the silver lining is that technology is improving. Modern digital cameras are increasing the sensitivity of digital sensors constantly. And each generation is better at handling image degradation, at handling digital noise while shooting at high ISO settings.
ISO sensitivity
The newest cameras boast impressive ISO range, from ISO 100 (which is the gold standard) to ISO 40-50k. This is great and exciting but we also need to get realistic. For each camera, there is an ISO setting beyond which the image quality becomes too poor to be acceptable. So know your camera and know its limits. The ISO settings and the range you will be using won’t necessarily be the ones advertised by the camera manufacturer.
Contrary to what happens with Aperture which has a connection to exposure and a creative use, Depth of field, and contrary to what happens to shutter speed, which has a connection to exposure as well and a creative use, Motion capture, the ISO setting has indeed a connection to exposure but I can hardly find any creative use.
The ISO setting on your camera should always be the lowest the scene you're photographing allows you. In modern cameras, my suggestion is to shoot with Auto (automatic) ISO, which lets the camera choose the ISO setting. The camera will always give you the lowest ISO value compatible with the Exposure Value, which is the combination of Aperture and Shutter Speed appropriate for that specific lighting condition.
If you have a Minimum Shutter Speed setting in your camera turn it on. If you don't, pay attention to your shutter speed and increase the ISO when you are shooting in low light conditions to avoid very slow shutter speeds that can cause camera motion blur.
In the end, follow a simple guideline: shoot with the lowest ISO number possible but don’t be afraid to bump up, to increase your ISO. It's better to have a picture with digital noise than a seriously underexposed or a blurred picture.
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