cheating ISO rating on DSLR cameras

Started Feb 21, 2013 | Discussions thread
Detail Man
Detail Man Forum Pro • Posts: 17,042
Re: cheating ISO rating on DSLR cameras

aired wrote:

if you have an accurate light meter you can find out much your camera cheats in the ISO rating.

The term "cheating" implies a rule being breached. Have a look at the following information below.

DxOMark's RAW-level "Saturation ISO" metric (sometimes referred to as "real ISO") relates only to RAW-level image-file scaling. Note that "ISO" ratings relate only to in-camera produced JPGs:

The Recommended Exposure Index (REI) technique, new in the 2006 version of the standard, allows the manufacturer to specify a camera model’s EI choices arbitrarily. The choices are based solely on the manufacturer’s opinion of what EI values produce well-exposed sRGB images at the various sensor sensitivity settings. This is the only technique available under the standard for output formats that are not in the sRGB color space. This is also the only technique available under the standard when multi-zone metering (also called pattern metering) is used.

The Standard Output Sensitivity (SOS) technique, also new in the 2006 version of the standard, effectively specifies that the average level in the sRGB image must be 18% gray plus or minus 1/3 stop when the exposure is controlled by an automatic exposure control system calibrated per ISO 2721 and set to the EI with no exposure compensation. Because the output level is measured in the sRGB output from the camera, it is only applicable to sRGB images—typically JPEG—and not to output files in raw image format. It is not applicable when multi-zone metering is used.

The CIPA DC-004 Standard (2006) referred to above can be downloaded for reading here:


bobn2 wrote:

REI is the only standard available when a camera has multi-pattern metering, and since they all do, that is what they all use. When you think about it, it is inevitable, since multi-pattern metering exists exactly to make exposure decisions at variance from using either 100% or 18% grey as fix points. Without standardising the operation of multi-pattern metering, there can be no EI standard except REI.


bobn2 wrote:

The ISO standard allows the white white point as well as 18% (the call them the saturation technique and Standard Output Specification, respectively). Assuming that all that happens in producing the output file is applying a 2.2 gamma, then the two will produce the same result. Of course in real life, manufacturers don't just do gamma correction, they apply S curves, sharpening, local contrast enhancement, saturation enhancement, all sorts of things, so the theoretical equivalency breaks down. Moreover, 'everybody else' doesn't use SOS, they use REI, which just allows them to set what they like. They have to do this because it's the only method allowed if the camera has multi-pattern metering, and they all do nowadays.

... the method that they are mandated to use is Recommended Exposure Index, which allows the manufacturer to use anything that gives a good result. In effect, under REI there is no standard.


Re: Please attempt to understand ISO 12232:2006

Andy Westlake wrote:

Ehrik wrote:

DPR unfortunately didn't notice that the GF1 ISOs are about one stop faster.

Oh no, not this again. The GF1 is about 1/3 stop more sensitive than marked, if you test it according to the ISO 12232:2006 SOS definition.

Remember, ISO is essentially a JPEG brightness measure, and really isn't about the raw sensor data at all - manufacturers are free to place metered middle grey wherever they like in the sensor's response. This is why DxOMark's non-standard definition gives unexpected results.


Note: DxO Labs explains their (DxOMark, RAW-level data only) "Saturation ISO" metric here:

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