Welcome to the Canon club! This high read noise is a perk of Canon sensors (among others).
With a perfect (ISOless) sensor, noise should be the same with the same exposure (counted in photons).
Yes, indeed (although I haven't tried the same test with my new R6 Mark II. It Sounds like you're saying don't waste the time). At least with my old Canon, it isn't ISO invariant.
I've been working on a (short?) paragraph or two on this; comments are welcome to get it smoothed out:
The term "
ISO invariance" refers to a camera's ability to maintain consistent image quality while capturing images at different ISO settings. Suppose a camera is (perfectly) ISO invariant at all ISO settings. In that case, it will produce the same level of image quality whether the ISO setting value that was set at capture or image brightness is adjusted in a raw converter. This ISO setting “doesn’t matter” in that scenario.
A non-ISO invariant camera may introduce more noise or less noise depending on the exposure due to this ISO setting; there is a variant to the outcome of setting the ISO.
In both kinds of systems, ISO will affect the recommendation of exposure (Aperture + Shutter speed), which affects the result of noise. In a non-ISO invariant camera, a higher ISO at optimal exposure can produce less noise than a lower ISO setting using the same exposure! This should not be the case with an ISO-invariant camera system. Hence the name and behavior. This behavior destroys the myth that “higher ISO produces more noise.” Underexposure is the result of (increased) noise, not ISO.
Altering ISO and, thus, the exposure recommendation can come in handy in low-light situations where you may need to increase the ISO setting and this exposure recommendation to avoid camera shake or stop motion with a higher shutter speed. Or if you wish to alter DOF, that wouldn’t be recommended* at a lower ISO setting. This results in less exposure and more noise. Again, it is worth stressing: underexposure is the cause of (increased) noise, not ISO.
* Lastly, the ISO provides a ‘
recommended’ exposure (Aperture + Shutter speed), but the savvy photographer does not need to accept this! Even without the inclusion of ISO, a savvy photographer will often ignore a reflective meter recommendation, for example, a black cat on coal or a white dog on snow. Such a recommendation would produce an overexposed or underexposed capture, despite the ISO setting in those examples.