ISO

ISO, or International Organization for Standardization, refers to a standardized scale that measures the sensitivity of a photographic film or digital sensor to light. In film photography, ISO indicates the film’s sensitivity, also known as film speed. A higher ISO value corresponds to a higher sensitivity to light, enabling the film to capture images in low-light conditions. Conversely, a lower ISO value indicates lower sensitivity, requiring more light to produce a properly exposed photograph. The traditional film ISO scale generally ranged from ISO 100 to ISO 3200. Modern digital cameras can have sensors with ISO capabilities in the hundreds of thousands.

Film ISO

The ISO rating of a film stock is determined by its emulsion’s inherent sensitivity to light, which is governed by the size and distribution of silver halide crystals within the emulsion. The larger and more densely packed these crystals are, the more sensitive the film is to light. However, this increased sensitivity comes at the cost of increased graininess and decreased resolution in the final image.

For example, a photographer shooting outdoors on a sunny day may opt for a low ISO film, such as ISO 100 or ISO 200, to achieve finer detail and minimal grain. In contrast, a photographer capturing images in dimly lit environments or requiring fast shutter speeds to freeze motion may choose a high ISO film, such as ISO 800 or ISO 1600, trading off some image quality for greater light sensitivity.

Popular film stocks with varying ISO ratings include:

1. Kodak Ektar 100 (ISO 100) – Known for its fine grain, high saturation, and sharpness, Ektar 100 is well-suited for landscape, nature, and outdoor portrait photography.
2. Fujifilm Pro 400H (ISO 400) – This film stock provides natural skin tones and excellent color reproduction, making it a favorite for wedding and portrait photographers.
3. Kodak Tri-X 400 (ISO 400) – A classic black-and-white film, Tri-X 400 is renowned for its grain structure, contrast, and latitude, making it suitable for a wide range of photographic applications.
4. Ilford Delta 3200 (ISO 3200) – A high-speed black-and-white film, Delta 3200 allows for shooting in low-light situations without the need for additional lighting, albeit with increased grain.

The term “ISO” in the context of photography is indeed related to the International Organization for Standardization. However, it specifically stands for the “ISO 5800:1987” standard for determining film speed. This standard combines the previous ASA (American Standards Association) and DIN (Deutsche Industrie Norm) systems into a single internationally recognized scale.

The ISO scale for film sensitivity is not limited to a range of 100 to 3200. While it is true that most commonly used films fall within this range, there are films with lower and higher ISO values. For example, some low-speed films have ISO ratings of 25 or 50, while high-speed films can go up to ISO 6400 or even higher.

Fujifilm Pro 400H has been discontinued as of January 2021.

Digital Camera ISO

The move to using ISO with digital sensors is a natural progression from film photography, as it provides a consistent and standardized way to measure the sensitivity of a digital sensor to light. In digital photography, ISO still represents the sensitivity to light; however, it is applied to the sensor rather than a film emulsion. Increasing the ISO value in digital photography amplifies the signal from the sensor, allowing the camera to capture images in low-light conditions.

In digital cameras, the sensor comprises millions of photosites, also known as pixels, which collect light and convert it into an electrical signal. This signal is then amplified according to the selected ISO value. As the ISO increases, so does the amplification of the signal, enabling the camera to record images in lower light situations. However, just as with film, increasing the ISO in digital photography also increases the noise, which manifests as graininess or speckled patterns in the image.

Examples of using ISO with digital sensors:

1. Landscape photography – When photographing landscapes in daylight, a low ISO value (e.g., ISO 100 or ISO 200) is typically used to minimize noise and capture high-quality images with a broad dynamic range.
2. Indoor or low-light photography – In situations with limited light, such as indoors or during the evening, a higher ISO value (e.g., ISO 800 or ISO 1600) may be necessary to capture a properly exposed image without using flash or additional lighting. However, this may result in increased noise in the final image.
3. Sports or action photography – Capturing fast-moving subjects requires a fast shutter speed to freeze motion. In such cases, a higher ISO value (e.g., ISO 800, ISO 1600, or even higher) may be used to allow for faster shutter speeds, even in well-lit conditions.
4. Astrophotography – Photographing celestial objects, such as stars and galaxies, often necessitates long exposures and high ISO values (e.g., ISO 3200 or higher) to capture faint details in the night sky. In these situations, the increased noise is a trade-off for capturing the desired level of detail.

In digital photography, the term “amplification” of the signal is an analogy to help understand the concept of increasing sensitivity. Technically, the process involves adjusting the gain applied to the signal from the sensor, which in turn raises the sensor’s sensitivity to light.

Noise in digital photography is not identical to grain in film photography, although they share some similarities in appearance. Noise results from the amplification of the signal, along with other factors, such as heat generated by the sensor. Grain, on the other hand, is a product of the film’s physical structure and the distribution of silver halide crystals.

In 2020, the video oriented interchangeable lens camera from Sony, the A7S III, offers a maximum ISO of 409,600.

ASA

In the film photography era, both ASA (American Standards Association) and ISO (International Organization for Standardization) were used to measure film speed or sensitivity to light. The primary difference between the two lies in their origins and the organizations that established the respective scales.

ASA was developed by the American Standards Association, which later became the American National Standards Institute (ANSI). ASA is a linear scale that directly corresponds to a film’s sensitivity to light. For example, an ASA 200 film is twice as sensitive as an ASA 100 film.

In contrast, the ISO film speed scale was established by the International Organization for Standardization. ISO 5800:1987 is the specific standard that defined film speed, combining the previous ASA and DIN (Deutsche Industrie Norm) systems into a single internationally recognized scale. The DIN system, originating from Germany, utilized a logarithmic scale to represent film speeds. A film’s DIN rating increased by three for each doubling of sensitivity; for instance, a 100 ASA film would have a DIN rating of 21°, and a 200 ASA film would have a 24° DIN rating.

The ISO standard for film speed combines both the ASA and DIN ratings, expressed as ISO [ASA value]/[DIN value], such as ISO 100/21° or ISO 200/24°. This unified system provides a comprehensive and internationally accepted representation of film sensitivity.