Hi everyone....I have been mulling over some tech stuff for actually about a year+ (quantum efficiencies, differences in sensors, how much head room really exists over my Canon 5DIV, etc).

But as I am working on my thoughts which include how ISO works, Tony Northup came along and stirred up the "ISO Invariance" pot. I am not sure he even knew what ISO Invariance was when he did his first video, as he just said "ISO is Fake." Fstoppers then did their own videos, and everyone seemed to finally notice iso invariance even though people have been talking about it for 2-3 years and I think DPReview has had a test for it for ~2 years.

If you are not familiar, ISO Invariance in short is the fact with modern cameras adding "gain" (increasing exposure, etc) in post processing is almost identical to increasing ISO while shooting. And if you watch Tony's or Fstoppers videos, you'll see them underexpose images by 4-5 stops, lift them in post, and have the noise be almost identical to a shot that was properly exposed in camera. Note, this is really about under exposing, as a blown highlight cannot be recovered and, not mentioned in the videos, there are still numerous benefits to properly exposing such as working with flashes and cutting down post processing, and, in my experience, I find some images are just best when dialed in in-camera (colors, feel, etc).

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). For high contrast scenes with a bright subject (loons, sunrises, etc) I often expose for the brighter part of the subject, underexpose the shadows, and bring them up in post. While ETTR still exists, it is less important (IMO) than it was 5-10 years ago.

But...all of this messed with my understanding of how ISO actually physically works. It is a bit funny that one thread is talking about old active users, as I am pretty sure my understanding of ISO and how it works came from Daniel Browning years ago. Maybe Neuro, or a combination.

My current understanding, camera sensors are charged diodes, and there are these basic steps:
  1. The camera diode/sensor is charged
  2. The charged diode releases electrons when hit by light
  3. The released electrons are then captured by the pixel
    (electrons captured per photon of light is quantum efficiency)
  4. In a CMOS sensor, the electrons are read by a transistor at the pixel level which creates an analog signal
  5. Gain can be added to an analog signal
  6. The analog signal is then converted to a digital signal
  7. The digital signal is recorded as part of the RAW file.
  8. Gain can be added in post processing


Some key things missing in the above that is relevant for some sensor discussions includes how the analog signal is read (in rows = rolling shutter; all at once = global shutter with obvious implications to on sensor circuitry/bandwidth) and where the analog signal is converted to digital (off sensor is the old method with more noise while on sensor has less noise as the signal does not have to travel as far).

So, with the above description, the question is, what is "ISO"?

Based on the TN and Fstopper videos and some online searches, what is being implied is that in camera ISO is simply an amplification to the analog signal in step 5. Whereas adding gain in post processing is simply adding to the digital signal in step 8. But Steps 1-4 never change while taking a picture.

Under this thinking, ISO invariance exists now because read/dark noise decreased to the point of being negligible when moving the analog/digital conversion on chip. In other words, since the digital signal is essentially as clean as the analog signal, it no longer matters very much where you add gain: in camera or in post processing.

Seems reasonable, I can find charts where that seems to make sense (DR/noise is linear with increasing ISO, etc).

But, it flies in the face of my understanding of what ISO is and I can also find charts where DR/noise are not linear with increasing ISO.

Quickly, my understanding of ISO was two things:
  1. The camera diode/sensor received different charges at certain key ISOs, so the charge in step 1 changes at certain ISOs
  2. Then the camera would add gain or subtract gain from the analog signal in this push/pull method that created this wavy DR/Noise graphs (step 5). Even today with a modern sensor like the A7RIII, there are two points from which it is linear, not one, implying two "base" ISOs.


But, I can't find anyone referencing variations in diode/sensor charge as part of ISO (variations to Step 1). It is all analog signal gain, or step 5 above.

So, after all that, the question is, does anyone know or know of a good reference for what ISO actually is? BTW, feel free to point out inaccuracies in the above. I am a Civil/Environmental Engineer, not an Electrical.

Thanks,
Brant

BTW, if you want examples:

Wavy results which to me imply either a variation in charge on the diode (step 1), or I actually see reference to a "dual gain" as explanation for the wavy behavior (so, a double step 5? or gain to analog and then gain to the digital signal?)
Name:  Read Noise 5DIII vs 5DIV.jpg
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The blue (5DIII) line shows the wavy behavior while the black (5DIV) sensor having a little, but a lot less waviness and much straighter (iso invariant).

Then Sony sensors have straighter lines (which I can see being gain added at a step 5), but the A7RIII this one drop (which I suspect is something else and have previously thought it was a charge difference at step 1):
Name:  Read Noise Sony.jpg
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If interested, as Nikon buys their sensors, the behavior of their sensors varies with the source. But you can see wavy behavior, or straight line with one drop.