ISO Mismatch - Stated vs. Measured

[munder]

Why is it that values for camera ISO settings are not even close to the measured ISO values?

I've seen that this generally seems to be the case regardless of the manufacturer or camera. For example for an ISO setting of 400 the EOS 5D Mark II is actually measured to be equivalent to ISO 285 film - 326 on the Nikon D3. Obviously these cameras can both achieve ISO above 285/326. Nikon's D2X(s) though is the one camera that seems to be right on the money. What gives?

[Bob Atkins]

Marketing most likely. You can get better noise results if the ISO is slighly lower than stated.

There may also have been some problem in that ISO for digital cameras wasn't well defined for a while. The ISO standard test methods only applied to film and involved standardized development and measure of gamma. It was difficult to apply these methods to digital.

Hpwever I suspect it's mainly marketing since actual ISO is always lower than stated ISO, never higher!

[munder]

It also makes it difficult to accurately compare one camera against another with regard to ISO and related characteristics such as SNR and dynamic range for those that don't have the resources to compare pictures from the cameras.

[whizkid]

 DxO Mark's raw data testing procedures offers the explanation that ISO values reported by the manufacturers are based on design choices. Principally to allow headroom they deem necessary to avoid oversaturation making it possible to recover from blown highlights. Apparently the complexity of all the variables and the tolerances allowed with the newer ISO specs for digital allow it.

[KeithB]

Seems like an awful lot of wiggle room for a "standard":
http://en.wikipedia.org/wiki/Film_speed

Quote:
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.
Unquote

Given that the "sRGB Color Space" is mentioned means that there is a *lot* of post processing going one.

[Bob Atkins]

Not only that but:

"The ISO standard 12232:2006 gives digital still camera manufacturers a choice of five different techniques for determining the exposure index rating at each sensitivity setting provided by a particular camera model"

So there is a ton of wiggle room. They probably also report the numbers as the nearest "Standard" ISO values (100, 200, 400, 800 etc.). This is somewhat similar to focal lengths where a lens that actually measures 380mm and is f6.0 is marked as being a 400/5.6 because it's within 5% of the measured value and those are the closest "standard" focal lngths and apertures. So if whatever measurement technique they chose comes out with an EI/ISO value of 330, they probably call it 400.

I'm sure it's no coincidence that the numbers are always moved to the high side, never the low side!

So basically it's a mess and the camera manufacturers can report any value for ISO they want as long as it's somewhere within a stop or so of an "equivalent" film ISO sensitivity.

When I tested the EOS 5D MkII, I found that to get equivalent image brightness, I needed to give the MkII about 1/3 stop more exposure at  the same indicated ISO then the original 5D, suggesting that the MkII "true" ISO was about 1/3 stop slower than the 5D.

[KeithB]

To be fair, and as a practical matter, unless you are using an external spot meter, it really does not matter what the ISO number signifies, it is only a guide to let you know the relative sesitivity level of the camera.  And a stop of slop is OK even for comparing cameras.  As long as a mfg. can't call an ISO setting 3200 when all other camera makers would indicate 100, things should work out OK.

[klindup]

It seems to me that much of what is said is marketing and very effective it is too.  One has only to look at the postings on various sites with questions from people who are moving to a higher spec camera when they have not mastered let alone exhausted the capabilities of their existing camera.  With regard to ISO values, I agree with the view that given most people rely on the camera to determine the exposure, the ISO value is more an index than an absolute value.  Surely the advice should be to learn what a given ISO setting on your camera means in terms of results.  It is another reason for not changing bodies every time the manufacturer brings out a new model.

Ken Lindup

[KeithB]

In "Mountain Light", Galen Rowell (excellent book, btw*) spends a chapter on how to get your camera to record exactly what you see.  One of the things he did was shoot his favorite film (I believe it was Kodachrome at the time) at various ISO's to understand exactly  how it responded to light and color.

* Though in the relatively recent (1999?) foreward he totally misses seeing the digital revolution coming.  The paperback is only $20 at Amazon - but uses Bob's Link!

[whizkid]

Can't remember the source or the accuracy but of the five ISO reporing methods allowed by the standards only two are most appropriate for digital. These are the newer of the standards added to the older three. It is lossy compression that the two newer ones address. It was stated that Japan requires either of those,  REI or SOS.