I have not read all the previous post and discussions about it, I say it in advance.
So I'm referring just to this phrase I quoted, extrapolated from the context.
If you ever used an incident light exposure meter, you would have seen that nowhere the "frame size" is ever taken into consideration; the EV (exposure value) of a scene is always the same, regardless of your gear. So if you "block" two sides of the triangle, the shutter speed and the iso (yes, "real" iso vary from brand to brand, and even between cameras of the same brand; let's pretend that iso is the same), than the aperture, when measured with an incident light meter, or the total light measured with a lux meter, it's the same thing, well the aperture/light intensity is exactly the same, regardless of your sensor size.
True, but not the point. Exposure meters don't take pictures, image sensors do. Exposure meters don't care about noise, photographers (hopefully) do.
Let me try to illustrate with two related examples.
APS-C camera: 50mm, f/2.8, ISO 400
FF camera: 80mm, f/2.8, ISO 400
The framing is the same (50x1.6=80), the DoF is thinner with the FF camera, the 'brightness' of the resulting image is the same (f/2.8, ISO 400).
APS-C camera: 50mm, f/2.8, ISO 400
FF camera: 80mm, f/4.5, ISO 1250
The framing is the same (50x1.6=80), the DoF is the same (FF stopped down to 1.3-stops to match DoF), the 'brightness' of the resulting image is the same (f/4.5, ISO increased 1.3-stops to compensate for narrower aperture).
Image noise is inversely proportional to total light gathered by a sensor. That's why the the FF R8 has a native ISO range up to 102,400, whereas the APS-C R7's native range tops out at 32,000 (and the iPhone 14 Pro tops out at 12,768) – manufacturers decide what is a maximum tolerable noise level and set the range accordingly (Apple sets a higher ISO cap relative to sensor size because of the heavy reliance of onboard, AI-driven noise reduction).
In the first example, the image noise is lower with FF. The ISO (gain) is the same, but the FF sensor is bigger so it gathers more total light. In the second example, the image noise is the same (the increased noise from the higher ISO used offsets the lower noise from more total light). Thus, in the first example the FF sensor is trading DoF for lower noise. In the second example, the resulting images are the same in terms of framing, DoF, 'brightness' and image noise – they are
equivalent.
The larger sensor allows you to achieve thinner DoF if you want it (and benefit from lower noise), but if you don't want the thinner DoF you stop down to get the the same DoF and simply raise the ISO and get the exact same image you'd get on APS-C. So, the larger sensor gives you more flexibility and more options. Importantly, one of those options is to get exactly the same image you'd get with APS-C.
If you're shooting in ample light (or adding your own) and don't need much subject isolation, there's really not much of an advantage to FF. But in more challenging conditions, FF has advantages – lower noise for working in low light (but you have to accept shallower DoF to get it), thinner DoF if you need maximum subject isolation (to match framing but get DoF as thin as f/1.2 on FF, you'd need an f/0.75 lens for your APS-C camera...good luck with that). In the example above, ISO 400 is not going to look meaningfully different on either sensor. But ISO 6400 on FF will look a lot better than ISO 6400 on APS-C. In that situation, you can have eyes in focus and ears not with a FF sensor...or you can have eyes and ears in focus with either sensor with the shot unusable due to high noise.
Put another way, FF gives you options that APS-C does not, in terms of the types of images you can obtain. A FF camera can do everything an APS-C camera can do in terms of DoF, shutter speed, ISO and noise, but it can also do more because it gathers more total light.
As I've already stated, the potential advantages of APS-C over FF are smaller/lighter systems, lower cost, and more pixels on target if you're focal length limited. That's it. Period. If you believe there are other advantages, you're arguing with optical physics...and you'll lose that argument. Every. Single. Time.
But as I've also stated, some people (far too many, IMO) are perfectly willing to ignore things like facts and physics and just assume their personal opinion is correct. It's clear that
@ReflexVE falls into that category, hopefully you do not and you've learned something today.