There is much confusion about what the ISO setting on a camera does and how its numerical values should be calibrated. The full answer is in ISO standard 12232 (2006 revision) and the Japanese Camera and Imaging Products Association [CIPA] standard
DC-004. The latter can be found at that link, but unfortunately the former costs US$123,
here so I have not seen all of it.
The CIPA requires that its members (all Japanese cameras makers) calibrate the exposure index settings (the "ISO" dial or menu setting) on their cameras using one of two measures, "Standard Output Sensitivity" or "Recommended Exposure Index". Both are based on brightness levels seen in end-products: getting "appropriate" levels in a camera's default JPEG output. Neither the ISO not CPIA standards care in the slightest how the intermediate raw levels are used in order to achieved this result.
ISO 12232 also defines several other measures of sensor performance, related to highlight and noise handling. Note that these measure the
sensor, not the camera as a whole, and in particular with a CCD (and some CMOS sensors), these measurements are on analog output signals in voltages or charges and are totally unaffected by any subsequent amplifier gain or ADC conversion. Thus
they are independent of the exposure index ("ISO") setting on a CCD camera. In particular, for a CCD, the saturation-based measure is the so-called base-ISO speed, which is about 100 for current CCDs with micro lenses and about 50 for those without. Things get more complicated with CMOS sensor, some of which apply different levels of gain to the signal while this signal is still on the sensor chip, depending on the camera's settings.
The ASA/ISO measure of film speed is based on adequate shadow handling, looking at handling of illumination levels four stops below the mid-tone level. The closest that ISO 12232 comes to matching this is in its two SNR-based measures, Snoise10 and Snoise40. (These are rarely discussed in "public" by camera makers of forum participants, but are important to technical users like astronomers, so are discussed into technical literature on sensors.) Roughly, these noise-based measures are given by the exposure index (level of sensor exposure) at which the signal-to noise ratio on the image of an 18% gray card is 10:1 and 40:1 respectively. For modern CCDs in MF cameras and backs, Snoise10 seems to be in the range 800-3200, limited largely by sensor dark noise, while Snoise40 is far lower and is limited by photon shot noise, since a SNR of 40:1 requires a signal of at least 1600 photons counted.
However, the measure that most people talk about around here (and often misunderstand because of its misrepresentation by DXO) is the sensor saturation based one "Ssat", which is what used to be referred to as base-ISO speed: this is a measure of the
maximum sensor exposure level, meaning
minimum exposure index that gives adequate highlight headroom: it is the sensor exposure level that sends an 18% gray card image to 12.7% of the full well capacity of a CCD (things got more complicated with some CMOS sensors.)
EDIT: I found parts of the ISO12232 standard, from which I quote (emphasis and bracketed comments are mine) it clearly state that the saturation based measure relates to overexposure latitude not "ISO speed".
6 Determination of ISO speed
6.1 General
… The maximum exposure level is the exposure level where typical picture highlights will be clipped as a result of saturating the image sensor signal capacity or reaching the camera signal processing maximum signal level. The minimum exposure level depends on the amount of noise that can be tolerated in the image. These situations lead to two different types of speed values, saturation signal-based values and noise-based values. The ISO speed is preferably determined using a noise-based method. [Snoise40] The saturation-based value is preferably used to indicate the camera's overexposure speed latitude. [Ssat] A second noise-based value is preferably used to indicate the camera's underexposure speed latitude. [Snoise10] ...
The next bit is rather technical, but the gist is that:
"ISO speed" should be reported as the value of Snoise40 (suitably rounded)
“ISO Speed Latitude" should be reported as the range from Ssat to Snoise10 (suitably rounded)
as indicated by my bracketed notes above.
6.4 Method of reporting
The ISO speed of a DSC shall be denoted ... “ISO xxx” … the reported number “xxx” shall be the value from the third column of Table 1 from the same row as the Snoise40 value (in the second column of Table 1) determined in 6.2. The ISO speed latitude shall be denoted “ISO Speed Latitude yyy - zzz D” ... The reported number “yyy” shall be the value from the third column of Table 1 from the same row as the Ssat value (in the first column of Table 1) determined in 6.1. The reported number “zzz” shall be the value from the third column of Table 1 from the same row as the Snoise10 value determined in 6.2.
TL;DR
- The saturation-based measure Ssat of sensor performance is in no way intended by the ISO as a measure of what the exposure index ("ISO") setting on a camera does, and in fact contradicts the CPIA requirements for calibration of that setting.
- For CCDs (and some CMOS sensors), Ssat gives exactly the same "base ISO speed" numerical value regardless of the exposure index ("ISO") setting.
- The saturation-based Ssat measure is totally unrelated to the ISO film speed; film speed is instead more closely related to the ISO's SNR10 or SNR40 measures, which typically given far higher values than Ssat.
- The ISO prefers that "ISO speed" of a digital still camera be determined and reported using the noise-based method Snoise40 … but I have never seen this done outside of technical publications!