“ISO” is the acronym for “International Standards Organisation”.ISO standards are recognized worldwide and conforming to them, although on a voluntary basis, is a quasi pre-requirement in many business domains.
In the field that interests us, i.e. photography, ISO refers to the sensitivity of the digital sensor to light. In the past, in the United States, film manufacturers were using the ASA (American Standards Association) scale and Europeans the ISO scale. Today, the two standards have been merged and it is the old ASA scale that has become the new ISO scale.
Film
At the time of the silver-based film, to change the film’s sensitivity , it was necessary to rewind and replace the film in the camera by a film cartridge of a different sensitivity, higher or lower, as needed. On some cameras, this could not be done halfway through a roll, so the photographer was stuck with the current sensitivity until he’d used all the film in the camera.
Click to enlarge: Film canisters of different sensitivity. Photo Montage © Charles Martel 2015
Digital
With the advent of digital technology, things have changed radically. ISO on a digital camera is something that’s dynamic and not fixed like a film’s. The sensitivity can be changed on the fly, either manually or automatically, during shooting and even for each image, if so desired.
Click to enlarge: ISO sensitivity scale on an advanced digital camera (Panasonic Lumix Fz1000). Image © Charles Martel 2015
Click to enlarge: Selecting the ISO on a Samsung smartphone. Image © Charles Martel 2015
Also, unlike film, the sensitivity of which was fixed in advance, it is possible to vary the sensitivity of the digital sensor in a much more gradual manner and in much smaller increments. So nothing prevents a digital camera from setting the sensitivity to ISO 178, 340, 1260, etc.
But that also depends on the specifics of each camera model. On most digital camera this is true only in automatic mode. But some advanced digital cameras offer the photographer the possibility of choosing a non-standard sensitivity manually.
This results in the fact that ISO is now part of the control tools available to the photographer to get the proper exposure, just like the aperture control or the exposure time.
Sensitivities Range
Each digital camera model is different from others by many aspects. One of these is the sensor used. It is this sensor that determines the range of sensitivities available.
Most digital camera operate in a range (or scale) of sensitivities ranging from ISO 100 to 6400 approximately. But some advanced digital cameras, equipped with a large sensor, can rise it much higher. And some may go down a little lower than 100, like 80 or 50.
Normally, the scale of sensitivities climbs or descends in 1-stop exposure steps. 1 stop difference is double or half the sensitivity according to the direction of displacement on the scale.
100, 200, 400, 800, 1600, 3200, 6400…+
But again, more advanced cameras generally give many more and smaller increments to this scale.
Effect of sensitivity on the image’s quality
The general rule in digital photography is: The higher the sensitivity, the more noise in the image.
Camera used: Samsung Galaxy 3S (Smartphone)
ISO 100 Example:
Click to enlarge: Flower à ISO 100 – Image © Charles Martel 2015
Enlarged Section (sky): Relatively low noise
Flower à ISO 100 – cropped – Image © Charles Martel 2015
ISO 800 Example:
Click to enlarge: Flower à ISO 800 – Image © Charles Martel 2015
Enlarged Section (sky): High noise
Flower à ISO 800 – cropped – Image © Charles Martel 2015
As you can easily see in the examples above, the noise is much more visible at ISO 800 than at ISO 100. However, again, the noise difference depends on the type of digital camera used, the sensor’s size, the processing applied to the image after the capture, etc. On an advanced Reflex type digital camera, with a large sensor, the difference would be significantly lower, if not invisible.
It is therefore in the interest of the photographer to minimize the sensitivity used, especially if the digital camera has a small size sensor. High sensitivity should only be used if the ambient light is low and the photographer cannot use a tripod to stabilize the camera, for example.
This is also why many serious photographers prefer to set the ISO manually rather than using the auto mode.
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© Charles Martel 2015