ISO simulates exposure
To demystify the concept of exposure—to make it approachable for beginners—many writers present inaccurate explanations of what ISO is and how it works. Unlike the aperture and shutter, which are physical mechanisms, the ISO describes an electronic function that simulates changes to exposure. ISO is not a variable of exposure because it doesn’t affect the amount of light the image sensor receives. Instead, ISO settings determine how brightly the camera renders a picture given the exposure you have set using the aperture and shutter speed. Thus, ISO lets you change picture brightness without further adjusting your aperture or shutter settings or, if possible, changing subject brightness by adding or subtracting light from the scene.
The standard ISO scale is easy to remember and follows a simple geometric progression: 50, 100, 200, 400, 800, 1600, 3200, 6400, 12,800, 25,600, etc. The available range will vary depending on the make and model of your camera, and, as with aperture and shutter settings, intermediate values are typically available. The difference in effective exposure between adjacent values is equivalent to a change of one stop. For example, adjusting ISO from 200 to 800 quadruples your effective exposure; switching from ISO 3200 to 1600 halves your effective exposure.
Many photography instructors describe ISO as an image sensor’s variable sensitivity to light. It’s a deceptively intuitive attempt at understanding the process but also categorically incorrect. There’s no commercially available, mass-produced image sensor with variable sensitivity to light. Unlike the photoreceptors in your retinas, which undergo chemical changes to become dark-adapted, image sensors’ chemical and physical properties remain unchanged when adjusting the ISO.
ISO: image sensors and amplification
Understanding how image sensors work will help you appreciate what happens when you adjust ISO. All image sensors are sensitive to a relatively narrow tonal range of light, known as the dynamic range or exposure range. Your exposure must fall within this range to ensure that visual information is recorded—not only faithfully and accurately, but at all.
An image sensor is a densely packed array of photosensors that detect light. Each photosensor, or pixel, will accumulate an electrical charge when exposed to light, and the strength of this charge is proportional to the intensity of light at each pixel’s location. Immediately after exposure, the electrical current generated by each photosensor passes through a signal amplifier. It continues towards the analogue-to-digital converter, which digitizes the signal, making it readable by the camera’s microprocessor.
It’s common to raise the ISO when achieving an ideal exposure by other means is not feasible. When you increase the ISO beyond the image sensor’s base sensitivity (typically1There are exceptions. For instance, the base ISO on the Nikon D850 is 64, and it’s 160 on the Fujifilm X‑T3. ISO 100 or 200), the camera underexposes the image sensor and then amplifies that signal to render an image of correct effective exposure. The extent of the image sensor’s underexposure is proportional to how many stops your ISO setting deviates from its base value. For example, if you take a picture using ISO 3200 and your camera’s base is ISO 200, the camera will render the final image from an exposure that’s four stops too dark.
Unfortunately, raising the ISO to increase picture brightness increases image noise. There are three types of noise in digital cameras: read noise, shot noise, and thermal noise. Read noise occurs when converting the analogue signal from the image sensor to a digital signal for processing. During this process, the signal is amplified and converted using electronics, which can introduce random variations in the measured signal.
Shot noise is caused by the random arrival of photons at the image sensor, which results in variations in brightness at each pixel during. Shot noise is typically highest in low-light conditions (darker exposures).
Lastly, thermal noise is caused by the random movement of electrons in the image sensor due to temperature fluctuations. It’s mainly problematic for photographers operating their cameras in hot environments, capturing long exposures, or shooting long video sequences. The higher the temperature, the greater the thermal noise, which can result in random variations in the signal.
Image noise is present in every picture you capture regardless of ISO; however, the signal-to-noise ratio (SNR) is highest (meaning less noise) at your camera’s lowest ISO setting because they require bright exposures. If you raise the ISO to maintain image brightness in darker shooting conditions, your camera amplifies a progressively weaker signal, which lowers the SNR (meaning more noise). Thus, increased image noise is the most apparent practical consequence of using higher ISO values. However, it’s essential to understand that on modern cameras, increased noise is caused by reduced exposures and not by increased amplification.