Quote:That, of course, is incorrect.
It is very valid for light.
An equivalent focal length, gives the same field of view.
An equivalent f-stop setting, gives the same depth of field.
An equivalent ISO setting, gives the same amount of light.
The "only" part is nonsense...
The whole single reason it is "related to sensor noise" is because equivalent ISO settings will form the image with the same amount of light.
Because you used the same amount of light.
Light per square area size is.... totally not interesting in any way. The paradigm has shifted going from film to digital. With film, when using the same film, it made some sense. When using two totally different films, not so much.
ISO is not based on amount of light per square mm. You made that up... Or just misunderstood things that way.
Iso: you're talking about something completely different, namely iso sensitivity of a sensor - that is totally irrelevant here.
Iso, to spell it out a little more than I did before, originally indicated film sensitivity, and with a certain iso and EV value comes a certain aperture and shutter speed range which is totally and utterly independent of the sensor, medium, or film size. You appear to say that it is not - well you're wrong. In short, it is related to the sensitivity of the medium and the related EV of the scene one is trying to photograph, whether the iso is artificial or not (in a way it is artificial with a sensor, other than base iso, but then, we could push film as well, be it much less so than a sensor base iso).
And this is entirely due to the fact that we are talking about standard quantities of light per standard unit as measured and used, with a measured EV and a set iso.
This has absolutely nothing to do with "equivalent iso". All that is important (if noticeable) is equivalent noise. And that is relevant, because to get the same magnification going from 4/3 to FF you need a 2x linear magnification, based on the light captured, which due to the actual size is 4x less, and hence carries 4x the noise, or two stops. That is the only "iso equivalence" that exists, and it really is noise equivalence, not iso equivalence. If we made the 4/3 sensor 4x bigger, same size area wise as a FF sensor, the noise would be the same.
- f-stop equivalence only has to do with DoF at the same size of print or image, but in effect is directly related to the real FL
- FL equivalence only has to do with the size iof the recording medium, film or sensor size, and is directly proportional to the crop factor
- iso equivalence when used as an amount of light is actually light units per square area units, because that is how light affects f-stop and shutter speed, not total amount of light per sensor or film size - using iso as total amount of light is incorrect in this regard
"noise equivalence", as I called it, depends on printing or displaying an image at the same size, and that is indeed influenced by sensor size, because a smaller sensor requires larger magnification to get to the same image size, and therefore will display more noise directly in proportion to the magnification and total amount of light received: (area in standard units) * (light units) received / (standard area unit). This is the only time total light received comes into the equation.
The latter, because we are talking quantities of light, we can express in f-stops, and hence in isos. All conditions staying the same, a 4/3 sensor receives a quarter of the TOTAL light of a FF sensor, and the image needs to be enlarged 2x linear (or 4x in area) to give the same image. This means that for the same size image a 4/3 sensor receives 2 stops less light than an FF sensor, which could be expressed as two iso grades, e.g., 100 iso FF and 400 iso 4/3.
However, this has absolutely NOTHING to do with actual measured EV for a certain ISO and f-stop and shutter speed used, and nothing with an "equivalent iso". If you'd say "equivalent noise levels", I'd agree, but only then.
IOW, what we knew already is even more true for 4/3: the lower the iso used, up to a point (base iso), the more total light will be received by the sensor, per area unit, and the lower the noise will be, but we do need to be careful to not overexpose, obviously.
Having said all that, it really is a moot point. Sensors, even 4/3 sensors, are way better than 35 mm film anyway in equivalent isos. This is mostly due to the fact that good amateurs got about 30 -40 lp/mm resolution from 35 mm film, and good pros about 60 lp/mm. We were already at this stage when we reached 6-8 MP and 12 MP respectively, and this is apart from everythign else we can do with digital as compared to film/analog.
In addition, what I also mentioned, if you do need thin DoF, and/or higher light intensity, use a Metabones adapter for your FF lenses, either a 0.71x Speed Booster, or a 0.64x one, which give you respectively a final crop factor of 1.4x and 1.28x (APS-H), and an f-stop increase of 1 stop and 1 1/3 stop. This also relates to the increase in teh amount of light reaching the sensor, and therefore the option to lower iso by 1 stop or 1 1/3 of a stop, and hence lowering noise levels (up to base iso).
Finally, what is really the point? I used to push 400 ASA/iso film to 1600 or 3200 in the past, resulting in a terrible amount of noise, even if we didn't call it that back then, and still printing good and acceptable photographs of 40 cm x 60 cm from it. But then, we didn't pixelpeep in those days, and we looked at pictures from appropriate distances (actually most "normal" people still do).
In the end, it is about the art, not about perfect pictures from a noise/DoF/FL POV - those are just means to get a result, to control the results we want to achieve, no more no less, and it serves no purpose to contnuously point out how bad everything is while we could do it the other way around.
A 20 cm x 30 cm print from a 4/3 sensor when captured and processed well, is better than anything we could do before the advance of digital, from a technical POV, and for me and many others that is more than good enough, considering the parameters, one of which is most definitely weight and size.
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....