Mastering Camera ISO: A Comprehensive Guide to Understanding Light Sensitivity

By valo | Valo | 31 Jul 2023

Demystifying one of the essential elements of photography

In photography, capturing the perfect image is all about finding the right balance between light and dark. To achieve this harmony, photographers must have a firm grasp of the three pillars of exposure: aperture, shutter speed, and ISO. This article will provide a comprehensive guide to understanding camera ISO, exploring its origins, how it works, and how photographers can harness its power to create compelling images.

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The Origins of ISO: Sensitivity in Film Photography

To understand the concept of ISO, it’s essential to look back at the early days of film photography. In the dawn of photography, photosensitive glass or metal plates were unable to react to light efficiently. Exposures used to be in the order of tens of minutes, even hours. In the late 1800s came the first sensitometer that could quantify how sensitive the emulsion was.

Over time, it was proven practical to define sensitivity in steps of two, i.e., equivalent exposure is achieved by doubling the exposure time while keeping all other factors equal. As time passed by, light metering devices were produced to help photographers.

Various grading scales of sensitivity were defined, including DIN and GOST, but eventually, the linear scale ASA took over. It later became the de facto standard that we use today after defining it by the ISO (International Organization for Standardization.

Films with a lower ISO number (e.g., ISO 100) were less sensitive and required more light to create a properly exposed image, whereas films with a higher ISO number (e.g., ISO 800) were more sensitive and could capture images with less light.

However, this increased sensitivity came at a cost: higher ISO films were more prone to producing grain, or tiny specks similar to digital noise, that could compromise the quality of the final image. Loss of shadow detail and exposure latitude was another drawback. As such, photographers had to strike a balance between sensitivity and image quality, selecting the appropriate film for their desired results and lighting conditions.

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The Digital Revolution: ISO in the Age of Electronic Sensors

With the advent of digital cameras, the concept of ISO has evolved to reflect the sensitivity of electronic sensors in the same manner as photographic film. In digital cameras, the sensor is composed of an array of photosites (or pixels), which convert incoming light into electrical signals. The intensity of the light determines the strength of the electrical signals, which are then amplified and processed by the camera’s internal electronics and converted into a digital image.

In this context, ISO refers to the amplification applied to the electrical signals generated by the sensor’s photosites. A higher ISO setting corresponds to a greater level of amplification, allowing the camera to capture images in low-light situations without significantly increasing the exposure time. However, as with film photography, higher ISO settings can result in increased noise in the final image. Both colour noise — unexpected colours registered by some pixels — and luminance noise — randomly brighter pixels carrying correct colour information. Computational noise reduction algorithms handle the problem with varying degrees of success.

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Understanding the Relationship Between ISO, Aperture, and Shutter Speed

As mentioned earlier, ISO is one of the three pillars of exposure, along with aperture and shutter speed. These three components work together to determine the exposure or the amount of light captured by the camera. To create a well-exposed image, photographers must balance these variables, adjusting one or more of them to achieve the desired result.

Aperture: The aperture is a variable opening within the lens that controls the amount of light entering the camera. A larger aperture allows more light to reach the sensor, while a smaller aperture limits the amount of light.
Shutter speed: Shutter speed, expressed in fractions of a second, determines the duration of light exposure on the sensor. Faster shutter speeds freeze motion and reduce the risk of camera shake, while slower shutter speeds allow more light to be recorded and can create motion blur.
ISO: ISO, as previously explained, influences the sensor’s sensitivity to light. A higher ISO increases the sensor’s sensitivity, enabling the camera to capture images in low-light situations. However, increasing the ISO can also result in increased noise or grain in the final image.

By adjusting these three variables, photographers can achieve proper exposure while balancing image quality and minimising noise. In many cases, it is desirable to maintain a low ISO to ensure optimal image quality. However, when shooting in low-light conditions or when fast shutter speeds are necessary to freeze motion, a higher ISO may be required to compensate for the limited available light.

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Native ISO, Base ISO, and Expanded ISO: Understanding the Range

When it comes to ISO settings, it’s essential to understand the distinction between native ISO, base ISO, and expanded ISO:

Native ISO: The native ISO range is the set of ISO values that a camera’s sensor can natively achieve without applying additional amplification or processing. This range typically offers the best balance between sensitivity and image quality, with minimal noise and optimal dynamic range. Some manufacturers actually report underestimates of the native sensitivity.
Base ISO: The base ISO is the lowest native ISO setting at which a camera can produce the highest image quality and the least amount of noise. For many cameras, this is typically ISO 100 or ISO 200. Smartphones tend to have even lower values. When shooting in favourable lighting conditions, using the base ISO will yield the best results.
Expanded ISO: Expanded ISO settings, sometimes referred to as “boosted” or “extended” ISO, are values that fall outside the native ISO range. These settings are achieved through additional signal amplification or processing, which allows the camera to capture images at extremely high or low sensitivities. However, the use of expanded ISO settings can result in a noticeable increase in noise or a decrease in dynamic range.

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Choosing the Right ISO Setting: Factors to Consider

Selecting the appropriate ISO setting for a given situation depends on several factors, including lighting conditions, desired depth of field, the need to freeze or blur motion, and the acceptable level of noise in the final image. Here are some guidelines to help you choose the right ISO setting:

In bright, sunny conditions or when using a flash, stick to the base ISO to minimise noise and achieve the best image quality.
When shooting handheld in low-light situations, use a higher ISO setting to increase sensitivity and maintain a fast enough shutter speed to avoid camera shake. Be prepared to accept some level of noise in your images.
If you need to freeze motion in low-light conditions or when using a telephoto lens, opt for a higher ISO setting to enable faster shutter speeds. Be aware that noise may become more pronounced.
In situations where noise is less of a concern, such as when capturing images for web use or small prints, feel free to use higher ISO settings to achieve the desired exposure and creative effects.
When shooting with a tripod, use a lower ISO setting and a longer shutter speed to minimise noise and maximise image quality. An exception is astrophotography because objects are very, very dim.

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Noise Reduction and Post-Processing Techniques

While minimising noise at the time of capture is ideal, sometimes it’s unavoidable. Thankfully, modern post-processing software offers powerful noise reduction tools that can help mitigate the impact of high ISO settings on image quality:

In-camera Noise Reduction: Many digital cameras offer built-in noise reduction features that can be applied during image processing. These features can help minimise noise but may also result in a loss of detail and sharpness.
Post-Processing Software: Applications such as Adobe Lightroom, Photoshop and Luminar offer advanced noise reduction algorithms that can effectively reduce noise while preserving detail and texture. Experiment with different noise reduction settings to find the right balance between noise reduction and image detail.

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Conclusion

Mastering the art of ISO is essential for any photographer looking to take full advantage of their camera’s capabilities. By understanding the relationship between ISO, aperture, and shutter speed, and learning how to select the appropriate ISO setting for a given situation, photographers can overcome challenging lighting conditions and capture stunning images. Furthermore, modern digital cameras exhibit less and less noise while their quantum efficiency increases from one generation to the next, typically thanks to using transistors of smaller sizes.

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