Exposure and ISO
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 mechanism and shutter mechanism, which are both physical parts of the camera, ISO describes an electronic function that simulates changes to exposure. ISO is not a variable of exposure because changes to ISO on its own, without additional adjustments to aperture and shutter values, do not 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’s light control and the shutter’s timing control. Thus, ISO allows you to change picture brightness without further adjusting your aperture or shutter settings or, if possible, modifying 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 varies 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. While this explanation may seem intuitively helpful, it is categorically incorrect. No commercially available, mass-produced image sensor has variable sensitivity to light. Unlike the photoreceptors in your retinas, which undergo chemical changes to become dark-adapted, an image sensor’s 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. When exposed to light, each photosensor, or pixel, accumulates an electrical charge proportional to the light intensity at its location. Immediately after exposure, the electrical current generated by each photosensor passes through a signal amplifier and continues to the analogue-to-digital converter, which digitizes the signal, making it readable by the camera’s microprocessor.
It is 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 (typically ISO 100 or 200), the camera underexposes the image sensor and then amplifies that signal to render an image with 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 at ISO 3200 and your camera’s base ISO is 200, the camera will render the final image from an exposure that is four stops too dark.
Unfortunately, raising the ISO to increase picture brightness also increases image noise. Digital cameras experience three types of noise: read noise, thermal noise, and shot noise. Read noise occurs when the analogue signal from the image sensor is converted to a digital signal for processing. This process involves signal amplification and conversion using electronics, which can introduce random variations in the measured signal.
Thermal noise results from the random movement of electrons within the image sensor due to temperature fluctuations. It poses significant challenges for photographers operating in hot environments, capturing long exposures, or shooting lengthy video sequences. The higher the temperature, the greater the thermal noise, which can create random variations in the signal.
Shot noise is caused by the random arrival of photons at the image sensor, leading to variations in brightness at each pixel. Shot noise becomes more pronounced with lower light exposure, such as when using high ISO values or underexposing your pictures. On modern image sensors, especially those with ISO invariance, read and thermal noise are minimal, and most of the noise effects seen in images are attributed to shot noise—the randomness of light’s arrival at the sensor.
Image noise is present in every picture you take, regardless of ISO; however, the signal-to-noise ratio (SNR) is highest (meaning less noise) at your camera’s lowest ISO setting because it requires bright exposures. When you raise the ISO to maintain image brightness in darker conditions, your camera amplifies a progressively weaker signal, which reduces the SNR (resulting in more noise). Thus, increased image noise is the most noticeable practical consequence of using higher ISO values. However, it’s crucial to understand that, in modern cameras, the underlying cause of increased noise is the reduction of light rather than increased amplification.
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