ISO is one of the most misunderstood concepts in digital photography. If you think ISO is an abbreviation for “International Standards Organization” or that it’s part of the exposure triangle, then you definitely need to read this article from beginning to end!
Unlike aperture and shutter speed, ISO doesn’t actually contribute to exposure when the image is recorded.
Let’s understand ISO from another perspective. ISO is just like the amplification of sound. The more you amplify it, the louder the sound and the less clear it will be. It can eventually reach a point where it is just noise and the words and music are indistinguishable. To avoid that, there are ways of turning up the volume which, like photography, depends on equipment.
ISO is not the “International Standards Organization”
To put the myth to rest once and for all, ISO is not an abbreviation for the “International Standards Organization.” The origin of ISO is actually a “word-forming element meaning ‘equal, similar, identical; isometric,’ from comb. form of Greek Isos ‘equal to, the same as’ (e.g. isometer ‘like one’s mother’).
ISO in Film Photography
To understand what ISO means in the world of digital cameras, it is helpful to look at film cameras.
The plastic emulsion of camera film is coated in delicate light-sensitive layers of silver halide crystals. Film emulsion that is rated as “highly sensitive to light” has layers of crystals that are coarser and larger. The larger grains allow the emulsion to record light more quickly. Likewise, film emulsion that is rated as having a lower sensitivity to light has a finer film grain.
The increase in size and coarseness of the film’s silver halide crystals results in an increase in the light sensitivity and the ISO speed (another term used in film). It results in pictures with more grains and lesser saturation, detail, and dynamic range.
ISO in Digital Photography
ISO Speed is a term that shouldn’t have progressed to the digital photography world because there is no such thing as digital sensor sensitivity. ISO in digital photography means something completely different—gain.
Gain signifies the increase in the image’s brightness after capture.
Let me explain.
The camera sensors consist of tiny units known as pixels. When we read the specifications on a camera, it may say that it has a 20 Mpix sensor, which means that it contains 20 million tiny units or pixels.
When we press the shutter and light hits the sensor, every single pixel measures the intensity of light by counting the number of photons reaching the pixel. The charge of the photons changes the voltage in each pixel; the voltage values are recorded by the camera. The RAW data is a collection of recorded voltage values from all 20 million pixels. The amplification of the signal happens after the data was collected by applying the gain.
For example, when you set the ISO to a value of 200, the camera still records the image at a base ISO of 100. When the image is recorded, the gain is applied to boost the brightness by a factor of two.
The ISO increase occurs at the cost of details, sharpness, and dynamic range. This part of ISO remains the constant in both film and digital photography because increasing the ISO has always come at a cost.
ISO is also now available at the change of a dial and today’s cameras are much better in terms of offering high ISO performance.
Another major difference between the digital and film worlds is the grain and noise. Film photography has grains; digital photography has noise. While film has grains in the highlights, digital has noise in the shadows. This is one of the major aspects that determine how we expose an image in the digital world - ETTR.
ETTR (Expose to the Right) vs ETTL (Expose to the Left)
ETTR refers to “Expose to the Right” and ETTL refers to “Expose to the Left.” ETTR means to slightly overexpose the image and ETTL means to slightly underexpose the image.
ETTL was popular in the days of film photography for the simple reason that grains were prevalent in the highlights. Therefore, exposure was shifted slightly to the left (generally 1/4th to 1/3rd of a stop) to produce a cleaner image devoid of grains.
ETTR is a digital phenomenon because digital noise occurs in the shadows and not in the highlights. The images are now slightly overexposed to reduce, minimize, or eliminate the noise factor in the shadows.
Using ISO to Control the Aperture and the Shutter Speed
The digital world of photography has opened up a variety of possibilities. Modern-day cameras are consistently breaking the barriers of low-noise performance, which aids both photography and cinematography. While the shutter speed controls the duration of time that light has to reach the sensor, the aperture controls the total amount of light that reaches the sensor; the ISO does not control the light.
You can, however, affect the aperture and the shutter speed for a particular shot using ISO. Shooting wildlife requires consistently high shutter speeds. Sometimes, when shooting in shades or during sunrise and sunset, the light is not always capable of providing sufficient exposure. Slowing the shutter speed introduces motion blur, which spoils the shot unless a motion blur is the desired effect. Instead, we can easily increase the ISO and keep the desired shutter speed without compromising the image we envision.
Similarly, while shooting Milky Way, it isn’t always easier to get a perfect shot at a low ISO value. The star trails are introduced into Milky Way photography if we use the shatter speed value above 25-30sec. To compensate, a high ISO can be used.
While it is established that ISO is not part of the exposure triangle in digital photography, it still affects the overall brightness of the image and helps us control the parameters (aperture and shutter speed) that directly affect the exposure. Some photographers are even creatively exhibiting their high ISO photography by using sensor noise as a tool!