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Comment by CWuestefeld | Hacker News Reader

ISO is basically a linear gain that's done on the sensor. As long as you aren't blowing out your photo and loosing information, it basically makes not difference if you do it in sensor or later while editing

This is true for some cameras, but certainly not all. Many cameras, especially pro or pro-sumer grade, have non-linear ISO. That is, there are ranges for which it behaves linearly, but typically there will be some range - say the minimum up to 1600 or something - where it behaves as a linear range, and then the next setting up from that (where the settings are typically 1/3 stop) will reset to a lower snr. (And yes, that does imply that in such cases it often yields better results to go up by one or even two clicks in ISO)

I'm not sure if there any camera-phones that behave this way, though.

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This is a great resource for finding out which ISO ranges various cameras are linear over:

https://photonstophotos.net/Charts/RN_e.htm

Taking one of the few recent-ish mainstream phones on this list since the subthread is about smartphone sensors: Samsung Galaxy S7 has an ISO range of 50–800 and basically all the noise values (measured in log2(electrons)) are between 2 and 3. There is a downward trend from 50 to ~300, above that it's all around 2. Other phones have similarly shaped graphs with different absolute values.

That sounds like the opposite of what GP (CWuestefeld) described. Am I misinterpreting the graph?

Lower sounds better to me, so the downward trend on a scale called "Input-referred read noise" sounds like it is tuning the Signal to Noise Ratio (SNR) on the sensor rather than just multiplying the sensor's output value, and it stops doing that above ~300 ISO. GP described that it would be a linear multiplier up until (for many cameras, not specifically smartphones) ~1600 ISO and after that it would be tuning the SNR. Do smartphones behave differently for some reason or am I misunderstanding something?

(It doesn't seem as though the absolute value says anything about the quality by the way, as a 10th gen Apple phone has a much lower value on this "noise" scale than a 12th gen one. The page does remark "raw values are not appropriate for comparing camera models because they are not adjusted for area", so this is probably that.)

I'm not an expert, but I think phone camera sensors really are that different from camera camera sensors, presumably (going out on a limb) because of tradeoffs they make to get good quality from small sensors. The sensors in top phones are about the same size as in the smallest cameras, and way smaller than in the cameras GP is thinking of.

> That sounds like the opposite of what GP (CWuestefeld) described. Am I misinterpreting the graph?

No. I was stupid in what I said above, getting the direction wrong. Where I said, "...will reset to a lower snr", I should have said HIGHER.

Ah, got it! Thanks for confirming! And it's definitely not stupid: I didn't even know of this concept, so I learned something today thanks to you :)

"then the next setting up from that will reset to a lower snr"

How does it magically make more photons fit the sensor..?

And why wouldn't you use that same magic at lower iso gain factors?

The base stops are ISO 100, 200, 400, etc.

Many cameras let you set the ISO in 1/3 stop increments, but if I recall correctly, many camera manufacturers just keep the sensitivity at the base stops and adjust the brightness via software.

So shooting at ISO 250 really means ISO 200 (underexposing what you requested) but then adding a third stop equivalent of brightening to the digital file. Conversely, using ISO 160 actually means the camera is using ISO 200 (overexposing) and lowering the brightness in software.

What this means, at least 10 years ago when I was more in tune with the photography world, is that people would prefer to shoot at the [base ISO stop - 1/3] levels to because those were the levels with the least noise near that exposure setting. The cost is you risk saturating more pixels in the highlights.

And for the same reasoning, the ISO setting s 1/3 over the base stops were typically avoided as they were noisier, albeit with slightly more dynamic range.

No magic and the same photons, but you can have the hardware sensor read them out differently. Specifically using varying amounts of analog gain / amplification before doing analog-to-digital conversion, minimizing noise. This varies based on camera design.

See the "ISO-Invariance and Downstream Electronic Noise" part here for a better explanation: https://www.lonelyspeck.com/how-to-find-the-best-iso-for-ast...

The article mentions the Sony A7S as an example, with the sensor showing marked improvements in SNR when reaching ISO 100, 200, 1600 and 3200, while behaving ISO-invariant wrt. noise in between those values.

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