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What is “ISO” in a digital camera?

ISO stands for “International Organization for Standardization”. It is a light sensitivity standard that was developed for film and continues to be used in digital cameras.

For both film and digital cameras, ISO is supposed to work the same. Each doubling of the ISO number corresponds to a doubling or halving of the light intensity or sensitivity.

Unlike digital cameras film cameras can’t change their ISO from picture to picture. The whole roll of film must be removed and changed out for a different sensitivity of film.

Digital Camera ISO

Digital Camera ISO is based on the camera sensor’s apparent sensitivity to light. The sensitivity changes are managed by an electronic circuit on the camera sensor. An increase in ISO causes an increase in the amplification of the signal from the sensor. Because the ISO setting affects the gain stage, it means that the sensor does not actually become more sensitive to light. What is actually happening at different ISO settings is that the signal is amplified by different amounts.

In digital cameras the sensor is not actually a digital device but an analog device. The signal from the sensor is stored as an actual electrical charge in a capacitor. The capacitor is then “read”, ie, discharged, and the energy stored within it is converted to a digital value.

Because of these processes there are certain limits to how sensor’s in digital cameras work. Digital camera sensors tend to have worse performance when the ISO is pushed higher than the base ISO. This means that the image tends to become more noisy at ISO’s that are far above the base ISO, and as a consequence of that, also have less dynamic range.

Film ISO

ISO was initially developed to standardize film sensitivity. Unlike digital cameras, film is an entirely chemical process. The light striking the film causes chemicals in the film to react to different frequencies of light in different ways that produces the different colors. Accordingly, the only way to control the sensitivity of the film is to change the chemical makeup of the film. Very high sensitivity film such as 1600 ISO or even 3200 ISO is very rare and hard to find these days. It’s not easy to make those films because the chemicals in them are highly reactive. Consequently most modern films are designed for the 100-800 ISO range of sensitivities.

When to use high ISO?

Typically, high ISO is used when high shutter speeds are required. These settings are particularly useful for sports photographers who want sharp pictures of athletes in the middle of a big play such as leaping for a touchdown pass.

High ISO settings can also be useful in reduced or low light situations. Although, if the scene has no moving elements it is much better to use a longer exposure at a lower ISO than it is to use a higher ISO.

Comparison of ISO settings

One downside of high ISO is often increased noise in the image. This is due to the fact that increasing ISO usually just increases the gain in the image.

Sensors tend to show different performance levels at different ISO’s.

Final Thoughts

Knowing a few things about ISO can help you make better decisions when taking pictures with your camera. Each camera may differ slightly, so knowing how your specific camera works will help you get the most out of it.

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