What is considered High ISO?

Question

So reading the questions I came across this Why do cameras include very high ISO settings even though they are so noisy? and my comment on this answer led me thinking, how do we qualify high ISO? Is it the top available ISO value? a percentile? Grain visible when viewed at a certian size?

Usually when speaking to someone you say high ISO, they know what you mean, you know what they mean and it doesn't need further explanation (to me high ISO is the highest setting I would be willing to go to and get a usable shot/the grain would be tolerable)

• On my old Canon 450D (crop, circa 2008, ISO range100–1600) I would consider ISO 1600 high ISO.
• On my Canon 7D (crop sensor, circa 2010, ISO range 100–6400) I would consider ISO 4000 high ISO.
• On my Canon 5D2 (FF sensor, circa 2009, ISO range 100-6400, expandable to 12800 and 25600) I would consider 12800 high
• On my Canon 5D4 (FF sensor, circa 2016, ISO range 100–32000, expandable to 51200 and 102400) I don't think I've got there yet (haven't needed to shoot above 25600, which was perfectly acceptable)...

When is something considered high ISO and more importantly is it quantative? Or is it all subjective?

Answer 1

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.

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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."

Answer 2

ASA Film Speed provides a simple enough answer for this. ISO 400 was considered standard high sensitivity film with anything above that being very high sensitivity film.

It would be easy to argue that anything above 400 ISO is High ISO. The real question is whether or not to include 400 in that or not. I would argue 400 should not be included as high ISO as its more of a standard. By many measures 400 ISO is the standard all-purpose speed which makes anything below it a Low ISO and anything above it (generally this would skip to 800 and above) would be High ISO.

Additionally, it is also recommended to begin with a general-purpose 400 speed film, such as Kodak Tri-X 400 or Ilford HP5 Plus, as these are some of the most flexible, user-friendly films available. ISO 400 film is ideal for handheld shooting in most outdoor conditions, many indoor conditions, and is also the most common film speed to push or pull during development.

Source: B&H Buying Guide: Film Photography Student

Further reading:

• http://www.photonhead.com/beginners/filmspeed.php
• http://www.guidetofilmphotography.com/film-photography-speed.html
• https://www.shutterbug.com/content/truth-about-iso-asa-film-speed-noise-and-fast-cars

Answer 3

High ISO is a traditional expression from the film days. High ISO is 800 ASA and up. It's always been that way. When you reached the 800 ASA film, you were buying high ISO along with the attendant grain. You had to be much more careful with your exposure and your process.

In digital cameras, there's a specific phrase called expanded ISO which is accessible but not recommended ISO. After that it's up to the camera manufacturer to decide what ISO they stop recommending.

On full frame expanded ISO usually starts at 6400 ISO and up (some specific models like the Sony A7S family might consider expanded ISO above 51200 ISO but that's very high).

For APS-C, expanded ISO is also in the above 6400 ISO range now but previously began above 3200 ISO.

For Micro Four Thirds, those little sensors are noisy from 1600 ISO with expanded ISO beginning as low as 3200 ISO.

Expanded ISO is finally really moving up though. The Fujifilm APS-C sensors in the X-T2, X-T20 and X-H1 hold together right through 12800 ISO. Expanded ISO as it's camera specific is a moving target.

But as soon as you are at 800 ISO you are into high ISO. Just go and look at the charts for your camera (and other popular cameras) at DXOmark and see what happens when to dynamic range and signal to noise when you reach 800 ISO and above in comparison to 50, 100 or 200 ISO.

400 ISO is the last ISO where most digital camera images still look their best. The drop off in dynamic range is already two full stops by ISO 800 for the Sony A6300. Signal to noise for the A6300 has gone from 44.4 to 36.4, already out of DXOmark's green zone. Color sensitivity holds up better but is below optimal perceptible levels by ISO 3200.

I've been careful to pick good and representative cameras here: the Sony A6300, Panasonic DMC-G80 and Canon 5D Mark III are considered among best of class for their respective categories (APS-C, MFT, full frame) and are all have been extensively used by pros and semi-pros. Outliers like the Sony A7S series don't change the rule. High ISO is 800 ISO and above.

Personally I won't shoot my Sony NEX-5T above 800 ISO, my Canon 5D above 1600 ISO or my Canon 5D Mark III above 3200 ISO. On each camera the deterioration in image quality is too perceptible at that point. Even looking at the Sony A7S which is tuned for ISO, a usable picture continues to exist at much higher ISO (say to 51200) but a lot of life has gone out of its picture by ISO 6400 (at that point, Sony A7S dynamic range has fallen from 13 stops to 10 stops).

High ISO, IBIS (in body image stabilisation) and fast autofocus should not be a replacement for good photographic technique. Tripods, pre-focusing and slower exposures still play a huge role in creating high quality images.

Answer 4

I don't think you'll ever get a majority to agree on what ISO is the lower margin for "high ISO". Cameras differ too much for that (we all know that photographers are rational beings...)
But as you say, most have a pretty good idea what it is.

But there are many factors that play a role. Of course, the sensor and camera electronics are omportant. But exposition also influences the result.

Then there's the final use of the image: you can get away with a lot of noise when all you have to do is show it on an average computer screen (it'll get somewhat averaged out when downsampling. If you have to show the same image as a double page spread (or an A3 print), you'll want the minimum noise possible.

Even the skill of the one doing the post-processing plays a role. With all the different options available, optimal noise reduction is not simple.

Answer 5

In the 1950’s I was a boy loading my first roll of color film, its film speed was 10. Since then, film speeds have climbed astronomically. Choosing the right film for a job is was no easy task. Most times your film holders and cameras were pre-loaded so changing film in the field was often prohibited. What I am saying, fellows and girls working this trade in the digital era have it easy. What ISO is high? Yesterday, optimum results dictated 100 ISO. Today it’s likely that 400 ISO will yield the same results. What happens when you elevate? Likely noise and some loss of scale.

The good news: Optimum ISO is a moving target because both the hardware and the software of the digital world evolve. I think the sky is the limit. On an early post I predicted that a noise free camera could emerge. This was criticized because physics tells us noise free is impossible. That may be true but the bottom line, is the level of noise present detectable by the observer? By the way, I label high ISO as being above 1000 (however ISO is a moving target).

Answer 6

I think you can go quantitative on it. Noise costs dynamic range. With ISO you get less and less dynamic range, and you can possibly specify when you have "enough noise to make it shitty" as a number - which is quantified.

Is it subjective? Well, the number may be, but again you can compare.

https://www.dpreview.com/news/4302149407/sony-a7r-iii-dynamic-range-improved-nearly-matches-chart-topping-nikon-d850

has a dynamic range chage for the A7RIII. Objectively - some ISO are worse than ofthers (like - why use 400 when 640 has more dynamic range?).

Objectively you can also say that if you have a specific quality in mind, and your camea is better, you can waste this dynamic range to get to the defined target quality.

So, while you can quantify it - the goal is subjective.

Answer 7

In the digital space, the term high ISO (the H marking under ISO) has often been used to describe what we more formally call expanded ISO or extended ISO. So under that definition, high ISO is any ISO setting in which the apparent brightness is obtained by multiplying the digital values read from the sensor, rather than by using an analog amplifier to raise the level of the signal.

The reason this matters is that digital values are quantized. Multiplying by two just means that the bottom bit (ignoring any dithering) will always be zero. You don't gain more information at the bottom, but you do through away information at the top (clipping). When you're converting to a lossy format like JPEG, extended ISO may still provide a real benefit, because you are going to be throwing away most of your bit depth anyway. When shooting RAW, however it is, at best, useless (and at worst, lossy).

The other common definition of high ISO, as others have talked about, is the point at which image quality suffers too badly from noise in lesser-quality cameras. This definition is entirely subjective and arbitrary, but I think of high ISO as anything above about 800. Better cameras do just fine way beyond that threshold; older/lesser cameras don't.

Answer 8

From my film experience, I generally consider high vs low ISO to relate primarily to shooting experience in relation to light and motion. Even though the ISO rating itself is quantitative, its selection is qualitative but carries certain risks.

Low ISO is useful when a narrow depth of field and/or motion blur is desirable. It carries the risk that camera shake could ruin the shot, necessitating the use of a tripod.

High ISO is useful when a larger depth of field and/or stopped motion is desirable. It also is useful for taking photos in low light without a flash. It also tends to not require a tripod except in low light or when using a long telephoto lens.

Traditionally, color quality and grain effects would lead photographers to carefully scrutinize their choice so they can minimize loss of quality. As noted so many times, however, digital sensors have reached the point where there is no practical loss in image quality for choosing a high ISO.

I am being intentionally vague about specific ISO values. Like any artist, the photographer needs to experiment and discover his/her own personal preferences for each scenario.

Answer 9

The ISO value is high when it allows fast shutter speeds in low light.

Answer 10

ISO is a sensitivity per sensor area, roughly corresponding to the previous ASA film values defined per film area. It combines with aperture numbers in order to make shutter speed depend on scene brightness, aperture number and ISO regardless of what kind of sensor/film crop/area you use.

For pixel-peeping at images, the noise per pixel is interesting. Since much of the noise level is inversely proportional to the arriving light energy per pixel, the area of a pixel is rather important in determining the noise levels. More modern sensors tend to have higher efficiency and lower inherent noise (not dependent on the actual arriving signal). Larger sensors with the same pixel number have larger areas per pixel and thus lower noise per pixel, higher pixel counts again increase the noise per pixel.

So what ISO level corresponds to what photon (signal-dependent) noise level depends on sensor efficiency, sensor size, pixel count, and the signal-independent noise level may or may not be ISO-dependent (that depends on the sensor technology) but will also depend on pixel area, sensor quality, current sensor temperature (which depends on ongoing use for live view and its update frequency, of course the ambient temperature, and total sensor area and heat conductance to the environment, possibly aided by fans).

From film times, ASA800 and above count as "high ISO": the film grain is a material constant that is independent from what light you actually throw at it. In contrast, a digital sensor's "grain" is defined by its pixel pattern (though some sensors "bin" pixels at "extended" ISO ratings, a technique that Fuji's X-Trans sensors formalize in a semi-religious manner). The resulting noisiness is not as much a matter of the underlying ISO than of the actual exposure you meter for.

In addition with the large impact of sensor size and pixel count, a single answer as satisfactory as "ASA800 and above" is illusionary since the ISO values of digital cameras are not tied to similar area constants like film grain. And even ASA800 film material has improved significantly over the decades, making that particular threshold change its impact over time.

Answer 11

The answer to this poor question is really simple. "High ISO" is an ISO value higher than what you use normally.¹ This has always been true and will always be true.

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¹ Obviously that's a fuzzy and moving target, something which David Richerby expressed nicely with his "horse and buggy" comparison.