What counts as “high ISO” in photography?

Question

People often say “high ISO” and everyone roughly understands it, but is there any objective definition? Does it mean the top ISO a camera offers, a certain threshold like ISO 800 or 1600, the point where noise becomes visible at a given output size, or simply the highest setting you personally find acceptable? Since different cameras handle noise very differently, is “high ISO” a quantitative standard or mostly subjective?

sensor noise iso camera-basics exposure

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

8y ago

user69369 · Accepted Answer

Being a man of science, I did some scholar-googling and came across some articles.

Too long, didn't read:

None of the scientific articles I found give a clear definition of "high ISO". However, they all link high ISO with higher noise levels. Therefore, I would say that high ISO is completely dependent on subjective criteria and the camera in question.

My sources:

From Tamer F. Rabie, "Adaptive hybrid mean and median filtering of high-ISO long-exposure sensor noise for digital photography" (2004):

Visible noise in a digital image is often affected by temperature (high worse, low better) and ISO sensitivity (high worse, low better). Some cameras exhibit almost no noise and some a lot and all the time. It has certainly been the challenge of digital camera developers to reduce noise and produce a "cleaner" image, and indeed some recent digital cameras are improving this situation greatly, allowing for higher and higher ISOs to be used without too much noise.

[...]

The resulting effect is the visibility of random noise artifacts in the acquired image that differs in severity from acceptable (at low-ISO settings ISO 2000) such that it becomes visually unacceptable.

[...]

One observation from the experimental results is the fact that different sensors exhibit different noise levels at the same ISO setting.

To me, that sounds like: "It depends on the signal-to-noise ratio (SNR), not on the ISO number."

From Y. I. Pyo, R. H. Park and S. Chang, "Noise reduction in high-ISO images using 3-D collaborative filtering and structure extraction from residual blocks" (2011):

Digital cameras are commonly used to acquire images in everyday life. Digital camera users want to get high-quality images at high-International Organization for Standardization (ISO) setting, in which ISO in a digital camera denotes the gain of image sensor. Images that are captured with low-ISO setting have little noise and vivid color with some blurring. On the other hand, images that are captured in low-light condition with high-ISO setting have higher gain of image sensor and are less affected by blurring, but more affected by noise than images with low-ISO setting.

[...]

Chrominance images have coarse-grain noise at high-ISO setting.

To me, that too sounds like: "It depends on the signal-to-noise ratio (SNR), not on the ISO number."

From Youngjin Yoo, HoCheon Wey, SeongDeok Lee, Chang-Yong Kim, "Profile based fast noise estimation and high ISO noise reduction for digital cameras" (2008):

In general, noise level is high in images taken by high ISO setting.

Once again, to me, it sounds like: "It depends on the signal-to-noise ratio (SNR), not on the ISO number."