What’s the difference between native ISO and unity gain ISO?
Asked 4/29/2016
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I’m trying to understand how native ISO and unity gain ISO differ on a digital camera.
I’ve seen native ISO described as the camera’s baseline sensitivity, where the sensor’s signal is handled without extra gain beyond what the camera is designed around. I’ve also seen unity gain ISO described as the ISO setting where 1 photoelectron corresponds to about 1 digital count (DN) after analog-to-digital conversion.
These sound similar, but people say unity gain is a single point while native ISO may be discussed as a baseline or range. Are they actually the same thing, or do they describe different parts of the imaging pipeline?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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Whether this is where you found it or not, the definition you give for "Unity Gain ISO" comes from the Clarkvision web site's section on digital sensor performance. As an updated version of that page explains, it's a silly concept. To quote:
The fundamental reason Unity Gain is not relevant is because the sensor in a digital camera is an analog system, not digital. The signals from the sensor are analog and only after amplification is the signal digitized.
By definition, the analog signal does not correspond directly to a "count". It's simply a level. My basic advice is to not worry about at all.
My slightly more opinionated advice is: if you are hanging out on forums where terms like this are bandied about and debated, get up, walk away, pick up your camera, and go out and shoot. If your images seem noisier than you'd like, get more light. If that's not possible, photograph something else.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
10y ago
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Generated from our catalog & community — verify before relying on it.
They are not the same thing.
Native ISO refers to the camera’s baseline operating point for the sensor/readout system — essentially the ISO setting the camera is designed around before additional ISO gain choices are applied. In practice, it relates to how the sensor’s full well capacity is matched to the camera’s readout and saturation limits.
Unity gain ISO is a different idea: it is the point where roughly 1 collected electron corresponds to 1 digital output unit (DN/ADU). That describes the relationship between the analog sensor signal and the camera’s digitized output.
Because image sensors are fundamentally analog before conversion, unity gain is not a deeply meaningful real-world threshold for image quality by itself. It’s just one possible way to describe sensor scaling, and many photographers consider it more of a technical curiosity than a useful shooting concept.
So: native ISO and unity gain ISO describe different things, and they do not have to coincide.
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