Why is minimum native ISO 200 on many mirrorless cameras?

Started 1 month ago | Questions thread
OP Charley123 Senior Member • Posts: 1,054
Re: It's a different ISO

Doug Pardee wrote:

And here we go again...

Once upon a time, cameras metered for 18% gray (I'm ignoring the arguments about the exact percentage). 18% was usually pretty good for black-and-white negatives, but color photos looked a bit dark. Back in the days when I shot color slide film, I typically added about +1 Ev of exposure compensation to get images I liked.

WIth the digital age, Canon threw out the 18% gray "standard." They brightened up the pictures by about one stop, through the simple approach of renumbering the ISO. What used to be ISO 200, they called ISO 100. That caused the metering system to expose one stop brighter. Nikon held out for a while, but eventually followed suit. When Sony bought up the Konica-Minolta line, they too went with the lower ISO numbering which gave brighter pictures.

Digital ISO as specified by ISO 12232 has two different standards for the manufacturer to choose from. "Standard Output Sensitivity" (SOS) ISO requires that the in-camera sRGB JPEG produce 18% gray when metering an 18% gray card... but there's an important exception I talk about below. "Recommended Exposure Index" (REI) ISO lets the camera manufacturer set the ISO to whatever gives exposures they think their customers will like.

Fujifilm, Olympus, Panasonic, and Ricoh/Pentax rate their ISO using SOS. If you manually set the exposure using readings from a handheld light meter, you'll get in-camera JPEGs that are exposed about the same as slide film would have been.

Everybody else, including the big three of Canon, Nikon, and Sony, rate their ISO using REI. If you manually set the exposure using readings from a handheld light meter, you'll get in-camera JPEGs that are exposed about about a stop brighter than slide film would have been.

So there you are. The same sensor that is rated at 100 ISO (REI) in a Sony is rated at 200 ISO (SOS) in a Fujifilm. [Do bear in mind that the one-stop difference is a matter of taste and can vary a bit.] Try it for yourself. Set your Canon to ISO 100 and your Fuji to ISO 200, then set shutter speed and exposure the same on both (manual exposure mode). The JPEGs produced by both cameras will be exposed about the same.

An important point:

SOS ISO only applies to "dumb" metering modes like average, spot, center-weighted, etc. (or handheld light meters). In-camera multi-zone metering (Evaluative, Matrix, Multi, ESP, or whatever) is specifically intended to deliver pleasing exposures, so it doesn't matter what the ISO numbering is.

If you take a Canon, Nikon, Sony, or whatever and snap an sRGB JPEG of an 18% gray card, it'll come out about 25% gray regardless of metering mode. But do that with a Fujifilm or any other SOS-rated camera, and you'll get about 25% gray when using multi-zone metering and about 18% gray when using a dumb metering mode.

I think a strong case can be made that Fujifilm, Olympus, Panasonic, and Ricoh/Pentax should join the other manufacturers in abandoning SOS ISO in favor of REI ISO. But they haven't. Yet.

You gave a great scientific and historical lesson-explanation. Thank you!

So all the digital cameras native ISO sensitivity really starts at approx SOS 200 ISO?

Is there a technological reason that native ISO of digital cameras can't start at a true SOS 50 or 100 ISO? Wouldn't that be an advantage on bright sunny days?

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