Quick Tips, our recurring feature explaining in simple terms certain key elements of photography, is back. Last time we took a look at the focus scale, how to zone focus, and when to use the technique. Today we’ll talk about one of the most basic controls in photography, ISO.
Many people just starting out are eager to learn the tricks of depth-of-field and motion-blur that come with understanding Shutter Speed and Aperture control, but completely overlook ISO. Even after years of shooting, some people still don’t know a thing about it. Even though ISO is part of the three key elements of photography, it’s something of a neglected “middle child” in the photographic family. ISO sullenly stews between Shutter Speed and Aperture, its more dynamic and interesting siblings.
This is unfortunate, as understanding and controlling ISO is just as crucial to making good images as Aperture and Shutter Speed, especially in the age of digital cameras. There’s no reason to be intimidated by ISO. The concept, once illustrated, is simple, and understanding it will improve every photographer’s images.
Typical ASA / DIN conversion table on a vintage camera. Film with ISO value of 200.
Brief History
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 ASA and DIN scales were used all over the world for many years, and conversion scales can be found on many of the most popular vintage cameras.
In the 1980s, these scales were widely superseded by the standards set forth by the International Organization for Standardization.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 ISO numbers. The scale used by analogue film is different compared to the scale used to show digital sensor sensitivity, but the practical principals of the two scales are the same regardless of their mathematical individuality.
What is ISO
In simplest terms, ISO indicates the sensitivity to light of a photographic film or image sensor. The higher the ISO number, the more sensitive a film or sensor is to light. What this means is that given an identical exposure of light over an identical period of time a film of ISO 800 will be more exposed than a film of ISO 100. 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.
Demonstration of the affects of ISO on two images with equal values for shutter speed and aperture.
While high ISO values allow for easier captures in low-light situations, they come with a cost. In photographic film, film grain is what makes up the image, 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. Yes, these high ISO images have more grain than whole wheat bread, and similar to baked goods, graininess is a matter of taste. In the past most people objected to grainy images, so photographers worked to avoid using highly sensitive films whenever possible. But grainy film can also be used to achieve artistic aims, and street photographers seem to love it for its sense of vintage grit.
Increasing the ISO setting of a digital camera comes with similar 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, digital noise is undesirable to most photographers, even more so than film grain.
Low-light image showing high-ISO noise.
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. In effect, an image taken at ISO 400 will require four times less light than an image taken at ISO 100, and therefore the shutter speed can be divided by four. When viewed this way, the importance of ISO becomes immediately clear.
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.
A typical menu for setting auto-ISO parameters.
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 affects 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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