What is ISO?
ISO is the name given to what’s commonly called ‘sensitivity’ in photography. It’s analogous to the system used in film but has a few fundamental differences, and these differences have been increasing over time.
Throughout this article we use the term ‘lightness’ to express how light or dark the final image is. This is to make clear that we’re discussing a representation on a white to black tonal scale, not a measure of emitted light. How ‘bright’ any tone specific tone appears to a viewer would depend on the display it’s viewed on.
At its most simple, ISO tells us that using specific exposure settings at a given illuminance level should give an image that looks like we expect it to. For many circumstances, this is all you need to know. It’s still a close-enough analogy for the film standard that a film-era light meter will still work for digital. Give or take.
But it’s often assumed that increasing ISO just adds amplification (voltage gain applied to the analog signal coming from the pixels), a bit like turning up the volume on an audio amplifier. This is not true, and this misunderstanding can make it harder to understand what your camera is actually doing. Virtually all modern cameras have at least one mode or function that diverges from the ISO = Amplification concept, so put that idea aside.
Seriously, what is ISO?
ISO in digital cameras (specifically ISO standard 12232:2019) is designed to resemble the ASA film speed standard that was adopted by the International Organization for Standardization in 1974.
ISO describes the response of the whole processing pipeline, relating exposure to end image lightness
However, there is a fundamental difference: the film standard defined the ‘speed’ of a film that would give a correctly exposed negative (for print film) when combined with a certain exposure and illumination level, but said nothing about how light or dark your prints would come out. The digital standard covers the response of the whole processing pipeline to give a final JPEG image with the ‘correct’ lightness.
|The ISO in digital photography is based on the lightness of the final JPEG. It doesn’t specify how this lightness should be achieved. It explicitly doesn’t specify a relationship between Illumination, Exposure and amplification for Raw files.|
To really understand the impact of ISO on your photography, it’s useful to recognize that it binds together a series of functions to relate exposure to image lightness:
- Exposure: the density of light hitting your camera’s sensor, based on the illumination level, shutter speed and aperture value used
- The inherent, unchangeable response of your sensor to light
- Lightening: the sum of all processing by which your camera delivers the expected image lightness from this exposure. ‘Lightening’ includes both analog amplification and any subsequent digital processing.
Separating out these elements helps us eliminate another error: that amplification or some aspect of the camera’s electronics increase noise when you raise the ISO level. In most circumstances, it’s the reduction in exposure that increases the noise, because it means you’re capturing less light and are more likely to see how random and noisy the light itself is.
How are you defining ‘correct’ lightness?
Before we go any further, it’s worth noting that there are two different definitions of ISO currently in use in cameras:
- SOS – Standard Output Sensitivity
- REI – Recommended Exposure Index
SOS is the simpler of the two. It essentially says that if you get an sRGB JPEG with lightness values of 118 (ie: middle grey) from a middle grey target shot with a given illumination level and exposure settings, then this must represent a certain ISO rating. This is the variant that most closely resembles the old film speed standard.
REI is the variant designed for multi-weighted metering modes. This is broadly similar to the SOS system, except it doesn’t specify the lightness of the final image. Instead the manufacturer gets to decide what ‘correctly exposed’ looks like. As a result, it can’t be measured, since there’s no definition of what the end result should look like.
It’s worth noting that that you cannot adjust the ‘sensitivity’ of a sensor.
A camera’s sensor will capture a certain proportion of the light that hits it, depending on the efficiency of its design. This can’t be increased or decreased. Everything after this step is a question of signal processing: you can try to minimize any further degradation, and you can manipulate how that signal gets represented in the final image, but only changing your exposure can adjust how much light you capture.
In most instances, the results from the two systems are usually pretty similar, but in a REI-based camera, you can’t take your light meter and conclude that following its advice will give you a specific image lightness.
Manufacturers are supposed to specify which of these standards they are working to, but this isn’t always done.
As an aside, the ISO 12232 document also specifies other ways of assessing camera response (based on saturation point or based on noise level), but these aren’t the ones your camera is claiming to comply with, so there’s no reason to expect them to align with what your files say in their EXIF.
What about Raw?
Raw is often talked about as being like a ‘digital negative’ but as we’ve seen, unlike the film standard, the digital version of ISO doesn’t specify what should happen in your Raw file.
The latest update to the ISO standard makes it explicit that it does not apply to Raw files. Until a tone curve is applied, a Raw file doesn’t have a ‘middle grey’: it’s up to the manufacturer to decide which Raw value should be used. Consequently, there isn’t a specific Raw value you can measure or check for correct lightness, so you can’t measure the ISO (or ISO accuracy) of a Raw file.
If you’re ever seen graphs plotting ‘Measured ISO’ against ‘Manufacturer ISO,’ then you risk being misled.
If you’re ever seen graphs of ‘Measured ISO’ plotted against ‘Manufacturer ISO,’ then you risk being misled. What they show is how a camera’s ISO settings are delivered, relative to an arbitrary system that assumes Raw files will saturate at a certain exposure. You can often work out how amplification is being applied using these graphs, but they do not show ISO accuracy, since the ISO standard doesn’t define a relationship between exposure and Raw saturation.
Now we’ve established that ISO includes exposure and lightening components, and that the lightening part is made up of more than just amplification, we can look at how these distinctions are exploited.
In part two of this article, we’ll look at how ISO is implemented in different cameras and different modes, why you tend not to get low ISO options in Log mode and how some video cameras are moving beyond ISO by separating the exposure and lightening aspects.
With thanks to bobn2 for sanity-checking the article prior to publication
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