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, and so on. 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 it’s 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, the chemical and physical properties of image sensors remain unchanged when adjusting the ISO.
ISO: image sensors and amplification
Having a basic understanding of 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 be set to 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. 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 and 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 in situations where it’s not feasible to achieve an ideal exposure by other means. 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 the number of 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 isn’t without consequence. At the point of capture, every image sensor is an analogue device with a base level of electronic noise that’s caused by minute variations in the structure of the photosensors and supporting electronics. 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. As you raise the ISO to maintain image brightness in darker shooting conditions, your camera increases amplification (or “gain”) of a progressively weaker signal, which lowers the SNR (meaning more noise). Thus, increased image noise is the most apparent practical consequence of raising the ISO.