Even though ISO is a major part of the three main factors that make or break a photograph, it’s something of a neglected middle child. ISO sullenly stews between Shutter Speed and Aperture, its more dynamic and interesting siblings.
But understanding and controlling ISO is just as crucial to making good images as Aperture and Shutter Speed, especially in the age of digital cameras in which ISO is so immediately switchable (with film, it was a single ISO per roll of film – more on that later).
There’s no reason to be intimidated by ISO numbers. The concept, once illustrated, is simple, and understanding it will improve any photographer’s images.
What is ISO
In the early days of photography a standard was sought to denote the speed of photographic film; that is, how sensitive a film is to light. The American Standards Association established a widely used system, as did the German Deutsches Institut für Normang. These scales were abbreviated ASA and DIN respectively, and they were used all over the world for many years (an obvious truth for anyone who’s looked at an old film camera on which the conversion scales can often be found).
In the 1980s, these scales were widely superseded by the standards set forth by the International Organization for Standardization. Today we follow this scale for measuring film speed, hence the abbreviation “ISO” and the numbers that follow. The value of the ISO used by film is different compared to the value used to show digital sensor sensitivity, but the practical principals of the two scales are the same regardless of their numerical individuality.
How ISO number impacts photos
In simplest terms, the light sensitivity of an image medium is denoted by an ISO number. With both digital sensors and film, the higher the ISO number the more sensitive that film or sensor is to light. What this means is that all things being equal (exposure time, aperture value, and environmental light of a scene) a film of ISO 800 will absorb more light than a film of ISO 100 and therefore show a brighter scene. The principal is the same with digital cameras. Setting a higher ISO number will increase the sensitivity to light of the camera’s sensor, while lower numbers will decrease that sensitivity.
While high ISO values allow for easier captures in low-light situations, they come with a cost. In photographic film, particles of light sensitive material suspended in an emulsion are what react to light and make the image appear after developing, so more sensitive films require larger grain. This use of larger grain makes each individual grain more visible, resulting in grainy photos at high ISOs.
Increasing the ISO setting of a digital camera comes with compromises to image quality. These digital anomalies manifest in the form of image noise. This noise presents as discolored pixels most visible in the darker areas of a low-light, high-ISO image. Similar to highly-grainy film prints of high ISO films, digital noise is undesirable to most photographers.
In the sample shots just below, my dog Cooper was laying in the same amount of low light. The settings of aperture and shutter speed remained the same through each photo – aperture set to F/1.8, shutter speed set to 1/60th of a second. The only change between the two shots was the ISO setting on my digital camera.
In the gallery just above, we see two film shots. The one on the left was made with Kodak’s super fine, low ISO film Ektar 100, the one on the right was made with Fuji’s high ISO Superia 1600. Note the difference in grain.
ISO normally starts at a base number, usually around 100 or 200, and increases through the scale by a power of two. A typical ISO sequence would run : 100, 200, 400, 800, 1600, 3200, 6400 and on (today’s digital machines can reach into the hundreds-of-thousands). With each incremental increase of the scale, the effective sensitivity doubles; so an ISO of 800 is twice as sensitive to light as an ISO of 400. This information may seem of little interest in practical terms until one realizes that this directly influences the amount of time required to capture a set amount of light.
Best ISO Practices
So how does a photographer know when to use low or high ISO? As a general rule it’s best to always use the lowest ISO possible in order to attain the highest image quality and cleanest detail. To get silky smooth shots, low ISOs are a necessity. In bright light, during studio shooting, or when using a tripod, keep the ISO set as low as possible. This will yield beautifully fluid images with rich blacks, and great contrast.
Handheld shots start to get wiggly at a shutter speed around 1/60th of a second, so when the light starts fading it’s time to turn up the ISO. Increase the ISO incrementally until the sensitivity is high enough to allow crisp captures with a relatively quick shutter speed. Modern digital cameras are so good at eliminating high-ISO noise that quality images can be produced even in the highest ranges, but the photographer needs to be aware that with every increase in ISO comes an increase in noise or grain.
One notable advancement in the digital age has been the advent of customizable Auto-ISO. This relatively new system is implemented in many modern DSLR and mirror-less cameras and it allows the photographer to dictate the behavior of the camera’s Auto-ISO. The shooter can now set the default sensitivity, limit how high the ISO is allowed to go, and the minimum allowed shutter speed. While setting these parameters intelligently can effectively solve the ISO problem, it’s still crucial to understand the principals of ISO. Not understanding the principals will make the chosen parameters nothing more than guesswork.
As with everything in photography, finding balance is key. Good photographers know how to manipulate the ISO setting of their camera to achieve the lowest grain or noise possible, while still allowing quick enough shutter speeds to capture sharp images. A complete understanding of ISO and how it impacts images is crucial to creative photography. This knowledge may be the difference between capturing a usable shot, or getting nothing at all.
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This post was originally published in 2014. It has been reposted after countless questions regarding ISO from new shooters. Hope it helps!