The Exposure Triangle – Understanding Sensitivity (ISO)

This is the third in a 3-part series on the Exposure Triangle. It is elementary information, aimed at those who are interested in learning more about the basic operation of adjustable cameras. Articles on aperture and shutter speed preceded it.

Understanding ISO

The ISO setting you select is the final piece in the exposure triangle. ISO setting has less dramatic impact than aperture and shutter speed on the final look of a photograph, but it’s something the serious photographer must understand. In very simple terms the ISO you select influences your camera’s sensitivity to light.

In film cameras, ISO (or ASA) 25, 64, or 100 were a common choices. Tiny particles of silver in film were the key element in capturing light. For shooting in lower light conditions, larger particles of silver were required, and these gave pictures a ‘grainy’ look. It’s incredible to think that modern digital cameras can produce fantastic results at ISO settings of 1600, 3200, and 6400 and higher. Some of the latest (2015) digital cameras can operate well at ISO 25,600. Additionally, with film, the entire role (24 or 36 exposures) were the same sensitivity. With digital, you can vary the sensitivity from frame to frame if you wish.

The sensor inside a digital camera effectively does the same job as film in a traditional camera. Adjusting the ISO setting influences the sensor’s sensitivity to light. Modern sensors are flexible and highly efficient at transforming light into an image, but they have their limitations. Increasing the ISO setting beyond a certain point introduces ‘noise’ into a picture and reduces the level of detail recorded. Some photographers refer to this noise as ‘digital grain’ and liken it to the grain of film. It is not grain, but, depending on the image, the introduction of this digital “grain” is not always a bad thing.

A good digital camera will have an ISO range of between 100 and 6,400. Each step between the numbers doubles the sensitivity of the sensor, so ISO 200 is twice as sensitive as ISO 100. All cameras have something called a ‘base ISO.’ This is the setting to select for the highest image quality, and is typically ISO 100 or ISO 200. If you’re shooting outdoors on a bright day, selecting your camera’s lowest ISO setting means you can make the most of the technology in your hands.

Once you understand the exposure triangle — the relationship between aperture, shutter speed and ISO — you can take much greater creative control of your photography. It can take years to master making the right selections, but it’s well worth making the effort. Your pictures can make the leap from snapshots to professional looking photographs once you’ve developed these skills. Everyone can take a reasonable shot with a smartphone or basic digital camera, but someone who understands the three elements of exposure and how they influence pictures can take outstanding pictures.

Some advanced digital cameras have a setting for the ISO speed to be adjusted automatically in response to lighting conditions. This is a very useful feature in some situations. For example, a sports photographer could select a fast shutter speed to catch the action and a wide aperture to isolate the subject from the background. The camera could then increase or decrease the ISO speed automatically to ensure a correct exposure. The photographer would need to check some shots and keep an eye on the level of noise in the pictures if shooting using this method.

The following are recommended ISO speed settings for typical subjects and situations a photographer might encounter.

1) Landscape photography on a bright day. Shooting at ISO 100 would ensure the best possible results from most cameras. Fine detail would be captured with little risk of any noise in the pictures. Shots taken could be enlarged for display without any loss of quality.
2) Portrait photographs taken in the shade. Avoiding bright sunlight is generally a good idea when shooting portraits outside. High-contrast and shadows created by direct sunlight aren’t flattering to the subject. Shooting in these conditions, ISO speeds of 200 to 400 would be recommended.

3) Shooting a sports game outdoors. Fast shutter speeds are essential when shooting sports like football. An ISO setting of 1000 would allow the photographer to shoot at shutters speeds of 1/500 to 1/1000. A modern digital SLR or micro four thirds camera would be ideal for this type of photography. Shooting with a smartphone or digital compact camera is likely to capture blurred images with high levels of noise.

4) Street photography in a busy city. The photographer needs to be able to react quickly to his surroundings for this type of photography. Experienced street photographers often prefer to get close, but also show some of the environment. They use relatively wide-angle lenses and often shoot, quickly, in different directions, encountering subjects in everything from bright lights to deep shadows. Setting aperture, for depth of field, and shutter speed, to freeze action, then setting ISO to auto, allows the sensitivity to automatically adjust to compensate for the fixed aperture and shutter speed. Longer lenses are often used to pick out subjects from the crowd, and for street portraits, so this means higher shutter speeds are needed to avoid camera shake. ISO settings of 400 or above generally would be recommended.
Once you understand the theory of aperture, shutter speed and ISO selection, the best way to develop your skills is to get out there and start shooting as much as possible. However, shooting is of little benefit unless you analyze your results, and consider if the resulting images match your vision for them. In other words, do the results look like you thought they would. If not, what settings could have been changed to get more predictable results?

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