IMO all the ISO story deserves much less attention than it receives. A good RAW shooter should simply study carefully how his camera responds to every amount of light, and forget about the ISO conventions. For me ISO values are just amplification gains, and I know I'll get the best overall DR and SNR captured if I can manage to do ETTR at the lowest real ISO. The ISO100, ISO200, ISO Low,... conventions are pretty useless, the only important thing is to know what's going on with the captured signal at every ISO setting.
For example: it's usually said (in the first place by the camera maker itself) that Olympus and Panasonic cameras lack ISO100, starting at ISO200. Well, I'd rather say Olympus cameras really HAVE ISO100 than saying they don't have it. When set at ISO200, a Olympus camera behaves closer to any other camera's ISO100 (specially old Canons) in terms of RAW highlight headroom, which in the end is what really matters. This means an Olympus RAW file from my E-P5 at ISO200 will have roughly the same RAW values than a RAW from my Canon 350D at ISO100 when both cameras receive the same amount of light per sensor surface unit (i.e. when the same aperture and shutter are set on both). If so, how the hell can you say the PEN doesn't have ISO100? or how can you say the Canon has ISO100?. When used with camera's suggested exposure at ISO200, Olympus cameras produce really bad SNR (an example of this is seen in so many users complaining at Olympus noisy skies), but the reason is not so much the M4/3 sensor performance, is that compared to other cameras you are sistematically underexposing by one stop!
Therefore I tend to ignore the "ISO100" naming convention, I just know what ISO setting I need to adjust to get the most DR of my captures and that's it.
Regards
Exactly. ISO-invariant sensors only have a few real ISO levels, with the rest being no different to pushing a lower-ISO shot in postprocessing. The Sony 42MP sensor only has ISO 100 and ISO 640. Nikon's D850 only has ISO 64 and 400.
Which leads to a particular quirk regarding the way non-base ISOs are treated. Basically, they're throwing away highlights. There's no disadvantage in shooting at USO 100/640 (Sony) or ISO 64/400 (Nikon), then pushing to a higher equivalent ISO, as opposed to shooting at a higher ISO in the first place. Conversely, many scenes where you'd want to shoot with a higher ISO are mostly dark, but with a few, small bright areas, which are captured well at ISO 100 (for argument's sake) but which are blown out at ISO 400. So it actually works out better to shoot at the base ISO and push, sparing the bright areas, rather than shooting at ISO 400, getting the rest of the image at the right exposure, but blowing the highlights. It would be great if there was a camera setting to
only shoot at the base, 'real' ISO levels, only using the other levels for the purposes of exposure simulation in the viewfinder. For instance, on the Sony, if you shot a scene at ISO 2500, you would see the image at ISO 2500 in the viewfinder, but the actual exposure would be captured at ISO 640, allowing for selective pushing to bring the image back to proper exposure, while sparing the highlights. At this time, there's no option to do this - if you want ISO 640, you have to shoot at ISO 640, which results in a completely black viewfinder (if using exposure simulation) or you having no idea what ISO you actually need (if not using exposure simulation).