Is this true? (Technical question)

[Doug Pardee]

A lot of misinformation in this thread.

F-stop

At the technical level, an f-stop does not guarantee any specific amount of light transmission. It is a measure of the effective aperture. To be silly about it, a lens made of the glass used in welder's goggles would pass much less light at a particular f-stop than a normal lens. In the movie industry, they measure each lens individually to determine a value called "t-stop".

At the practical level, that variation just isn't enough to be important for still photography. In practice it's never going to be anywhere close to the 1 stop difference that this other shutterbug was claiming. So we're quite happy using f-stops as an estimate of light transmission.

ISO

ISO ratings for digital cameras are not based on any scientific measurement. There is no guarantee whatsoever that two cameras with the same lenses, same shutter speed, same f-stop, and same ISO setting will get the same result from the same scene.

Recent digital cameras are rated using ISO's "REI" (Relative Exposure Index) process, which can be summarized as "whatever the manufacturer feels gives good-looking pictures". This is the only ISO rating process that is available when pattern metering—matrix, evaluative, multi-zone, or whatever the particular manufacturer likes to call it—is used.

In a few cases, you'll find cameras that are rated using ISO's "SOS" (Standard Output Sensitivity) process, which does provide a measurement standard for non-pattern metering… but only for JPEGs produced in-camera. This will almost always be a higher ISO number than what the camera maker would have assigned as REI, and thus will produce darker pictures—too dark for most tastes.

Older digital cameras were rated using an equivalent of the REI process, but technically the numbers are not ISO because at the time there was no ISO standard available for rating them. The REI process was introduced to the ISO standard in approximately 2007.

There is not, and never was, any measurement-based ISO standard for rating response when using pattern metering.

There is not, and never was, any measurement-based ISO standard for rating images produced in Adobe RGB 1998 color space (or any other color space except for sRGB).

There is not, and never was, any ISO standard at all for rating RAW response. ISO standards only apply to images converted in-camera.

Until approximately 2007, there was no ISO standard at all for rating JPEG response.

The older ISO standard provided measurement-based standards for camera-produced uncompressed sRGB image files (essentially TIFFs) with non-pattern metering. That older ISO standard was almost never used for consumer digital cameras.

More information on digital camera ISO from these links:
Overview: Wikipedia
Technical details: Doug Kerr (PDF)