ISO Invariance

[HDNitehawk]

But that said, the natural conclusion to ISO invariance is that you can under expose certain modern cameras with little penalty, and I do use this in my photography and this is one of the reasons I bought the 5DIV (that and f/8 AF points, etc).

So wouldn't this make sense?

If your camera is set on the exact same shutter speed and aperture the sensor gathers the exact same amount of data regardless of the ISO setting.
If you adjust your picture to the proper exposure in post the data was always there, you would only be changing the point in the process it is done.

1. The camera diode/sensor received different charges at certain key ISOs, so the charge in step 1 changes at certain ISOs

I have never heard that one, why would it?

I do remember old conversations or debates about noise. A certain amount of noise is introduced at the sensor, additional (spatial) noise when you increase gain.
So perhaps if someone could explain how increasing the gain works, does it change the signal to noise ratio?

Engineers:
https://www.sis.se/api/document/preview/907324/
https://webstore.ansi.org/Standards/...SAAEgLtPPD_BwE

[Kayaker72]

Interesting review... I'm fascinated by this stuff although most of it is way over my head (mechanical engineer, barely passed my mandatory electrical course 30+ years ago). I'd like to know where noise comes into play in all this. What is the intrinsic flaw (if that's the right word) in the electronics that creates noise? Do the diodes become less accurate as charge increases with ISO?

Yeah....this Civil Engineer is breathing through a straw on a lot of this.

First, I am becoming convinced that I either misunderstood or whoever suggested ISO was based on varied diode charge was wrong. To me, it does make sense as increase charge would be more sensitive (fewer photons in low light but more electrons to get bumped off the sensor), but also more potential for noise (random electrons getting bumped off, more heat, etc). But, I even came across this reference where myth #1 on clarkvision states that "ISO changes sensitivity" is false. Then, i

n all the articles I have read that actually look this granular (which actually isn't many), I only see references to ISO being adding gain to the analog signal and that the diode/sensor to pixel/photon wells are the same for every ISO setting (i.e. base ISO). So, as the analog digital conversion has become more efficient (granted, only for some sensors), you get ISO Invariance.

Overall, it sounds like the "myth" of changing sensor sensitivity has been out there, I picked it up someplace.

It looks like the article I like for noise is no longer on the web (at least my link doesn't work). But in terms of understanding what is happening on the sensor level, this video is great (also talks about film, which is fun):

https://www.youtube.com/watch?v=MytCfECfqWc

Then I also like this article:

http://dpanswers.com/content/tech_iso.php (old...it is actually one of my first saved links for photography)

I came across this article while checking my understanding of ISO and ISO-Invariance. But it has a nice discussion of noise part of the way down:
https://photographylife.com/iso-invariance-explained

But, generally, there is noise associated with the variation of light (i.e. shot noise) that has nothing to do with our cameras, but then there is noise introduced during exposure, with inaccuracies in the voltage readout, while the analog signal is transferred (this is what plagued/plagues Canon), and then during the analog to digital conversion (again, any inaccuracies, electrons being added subtracted, etc). Overall, anything within the camera is generally termed "read" noise.

So wouldn't this make sense?

If your camera is set on the exact same shutter speed and aperture the sensor gathers the exact same amount of data regardless of the ISO setting.
If you adjust your picture to the proper exposure in post the data was always there, you would only be changing the point in the process it is done.

Yes, this makes sense for a truly ISO invariant system. However, in older systems and some modern systems, they are not iso invariant. For example, with ISO being added to the analog signal, all noise that happens while generating the analog signal is also increased. But all noise added while transferring the analog signal (in some cases, off chip) and while converting the analog to a digital signal is not amplified. Thus, with the exact same exposure settings you would get less noise in your RAW file if proper exposure was ISO 800 by setting ISO in camera to ISO 800 (adding gain to the analog signal) rather than shooting at ISO 100 (base ISO for most cameras) and adding 3 stops of gain post digitization/creation of the raw file. ISO invariance, the noise added after creating of the analog signal is negligible, so you could shoot ISO 100 and increase in post and have basically the same image. This is essentially what TN and FStoppers did.

For years, the knock on Canon sensors was that their analog to digital conversion occurred off chip, thus there was significant noise added while transferring the analog signal off chip. Also, I've seen write ups that part of the "1D" treatment, was multiple analog-digital converters, thus faster, less heat generated and less noise. But, starting with the 80D and including the 5DIV/1DXII/M6/M5/M50/R (maybe others), my understanding is that these cameras have on chip AD conversion, thus less transfer and less noise. These cameras, when you look at their noise/DR results, are pretty linear. In contrast, the 6DII seems to still be off-chip conversion. Still a great camera, but this is why some on the internet bash the 6DII.

I have never heard that one, why would it?

Yeah, I am pretty certain I picked up the "myth" somewhere. But, I picked it up, it made sense to me, and I saw data that fit the model (wavy noise results with varied ISO/quick change in noise results). But now, I can't find a single reference supporting this idea and have found several that seem to counter it.

So, I have areas of expertise, this is not one of them. It is always good to have some fluidity in your thoughts and opinions. Here, I am moving off this idea that different charges can be applied to a sensor and that contributes to ISO.

To explain the data, there has to be more to ISO than just gain added in one step to the analog signal. "Photons to photos" references dual gain to explain the wavy feature. Anyway, I'll post if I ever figure it out.