When I started taking photographs on film, film speed was described using terms such as slow, medium and fast, with ratings including DIN, ASA and ISO. ISO is still used in digital photography, although the way a digital camera responds to a setting is different from the sensitivity of film.
There is no single ISO setting that suits every situation. The aim is to choose a value that gives you the shutter speed and aperture you need, while keeping image quality at an acceptable level. This article looks at the trade-offs, as well as base ISO and Auto ISO.
The exposure triangle
The exposure triangle illustrates the relationship between ISO, shutter speed and aperture. Changing one setting affects the exposure, unless you compensate with one or both of the others.
- Aperture: A larger f-number means a smaller aperture opening. Less light reaches the sensor, while depth of field generally increases.
- Shutter speed: A longer exposure allows more light to reach the sensor, but also records more movement. A faster shutter speed freezes action but allows less light in.
- ISO: A higher ISO setting allows the camera to produce a brighter image from less captured light, but usually increases visible noise and can reduce dynamic range.
A simple ISO guide
These ranges are only rough examples. Different cameras handle high ISO settings differently, and the right choice depends on the scene and the result you want.
Low ISO
Low ISO settings, often around ISO 50–200, are useful when image quality is the priority and there is enough light. They generally provide lower noise and greater dynamic range than higher settings, although the exact behaviour depends on the camera. A low ISO is a good choice for landscapes, studio work and long exposures where you want to preserve detail.
The trade-off is that you may need a slower shutter speed or wider aperture to achieve the correct exposure. If you are handholding the camera, a slow shutter speed can introduce camera shake. A tripod or image stabilisation may help, but neither can freeze a moving subject.
Medium ISO
For many cameras, ISO 250–800 is still a very usable range. It gives you more freedom to choose a faster shutter speed or a smaller aperture without a dramatic increase in visible noise. The actual point at which image quality begins to deteriorate depends on the sensor, exposure and how the photograph will be used.
High ISO
Higher ISO settings are useful when the light is limited and you need to maintain a fast shutter speed. I rarely wanted to go much beyond ISO 6400 with my Sony Alpha A99V, but that was a personal preference rather than a universal limit.
Noise is not always the enemy. In high-contrast black-and-white photography, a little texture can contribute to the mood. What matters is whether the noise distracts from the image. A sharp photograph taken at a high ISO is often preferable to a blurred photograph taken at a lower one.
Shot at 1/1000 sec, f/2.8, ISO 500 with a Sony Alpha A99V and Minolta 80–200mm HS APO G at 160mm.
Base ISO
Base ISO is the camera’s native reference sensitivity, where the sensor typically offers its best combination of image quality and dynamic range. Some cameras also offer extended low ISO settings, often marked LO. These may be achieved by processing rather than by operating at a lower native sensitivity, and can reduce highlight headroom. My Sony Alpha A99V has a base ISO of 100.
Practical use of ISO on the Sony Alpha A99V
The combination of the A99V’s full-frame sensor, the Minolta 80–200mm HS APO f/2.8 and Sony’s SteadyShot image stabilisation worked very well for me. I generally tried to stay below ISO 800, but I was happy with many photographs taken at ISO 1600. For night photography, I went higher, sometimes as far as ISO 6400, particularly when working in high-contrast black and white.
Auto ISO
Most modern cameras offer Auto ISO, which allows the camera to adjust ISO as the light changes. It can be very useful when you want to maintain a minimum shutter speed while retaining control over aperture and exposure compensation.
Auto ISO does not know what level of noise you are willing to accept, so it is worth setting an appropriate upper limit. On the A99V, the Auto ISO range can be configured through the camera’s ISO settings. The menu names and options vary between camera models.
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