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11-19-2011, 02:52 PM   #1
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Iso consistency

As I understand it, increasing iso will change the electronic gain that is applied to the captured signal (number of photoelectrons), so that a similar image brightness can be achieved for less exposure time (same aperture value); e.g., going from iso 100 to iso 500 should require 1/5 the exposure time.

That makes sense for iso values within the same camera, or for cameras with the same sensor, but what about different sensors and formats with different light collection efficiencies?

In film, the ASA number implied a specific exposure requirement to achieve a good image, and (I'm assuming), this was standardized across all film producers by some independent testing agency so that ASA 400 from Fuji, Kodak, etc. all responded similarly.

How does this work in the digital world?

Any comments appreciated.

11-19-2011, 03:25 PM   #2
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This is a matter of interest to camera testers, as well as standards organisations.

DXO Labs has some discussion that seems apropos your question:
DxOMark - Pushed ISO: Let's make it clear

QuoteQuote:
Now, let’s track back the elements that determine the exposure in the final picture. A first key factor is the sensor exposure: How much light falls on the sensor? Sensor exposure is determined by several factors, at least two of which cannot be adjusted by the photographer :

- The illumination of the scene (except for studio photography)
- The transmittance of the lens (see more)

Others are selected by the photographer or the camera auto-exposure system in order to get a correctly exposed photograph:

- Exposure time and lens aperture (These parameters are also constrained by other conditions as the motion of the scene and the desired depth of field)

There is a last parameter that is almost orthogonal to the ones above and which allows the photographer to obtain a good exposure: ISO sensitivity. Before digital photography, ISO (or even ASA) was directly related to the chemistry of the film. In the digital era, it is now possible to (almost) seamlessly change the sensitivity of the sensor. Therefore, when exposure time is limited by motion blur, and the lens is already wide open and the scene is dark, it is still possible to obtain a well-exposed picture by playing with the ISO setting. The only limit is that with higher ISO comes also more noise, which eventually makes the image too bad to be used.

For a given scene illumination (measured by a luxmeter), there are a number of combinations of exposure times, lens apertures, and ISO settings that lead to the same exposure. Fixing two of them determines the third one.

There are other elements of exposure that happen backstage, unknownst to most users. The image captured by the sensor (the RAW image) is processed by a complex image processing pipeline (the RAW converter) that is embedded on the camera or exported to a computer. The picture exposure can be digitally modified by the RAW converter via amplification gains, tonal curves, etc. Processing the same RAW image using different RAW converters usually leads to photos with different exposures.
Further,

QuoteQuote:
Definition


ISO sensitivity (also known as ISO speed) is a numerical value calculated from the exposure provided at the focal plane of a digital camera to produce specific camera output signal characteristics.

ISO Standard 12232 defines two ways to measure ISO sensitivity. The first relates sensitivity to the exposure necessary to saturate the camera. The second, seldom used, compares the relative exposures to obtain different signal-to-noise ratios. The more common saturation-based method is described below.
then follow the formulas.
DxOMark - ISO sensitivity

So the ISO specs do take into account varying sensor, and lens, efficiency.

You might enjoy browsing around some of DXO's technical articles for lots more info and discussion:
DxOMark - DxOMark Insights
11-19-2011, 09:17 PM   #3
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Thanks RAWR, that's the information I was looking for.
11-21-2011, 09:59 PM   #4
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QuoteOriginally posted by PENFRED Quote
In film, the ASA number implied a specific exposure requirement to achieve a good image, and (I'm assuming), this was standardized across all film producers by some independent testing agency so that ASA 400 from Fuji, Kodak, etc. all responded similarly.

How does this work in the digital world?
ASA hasn't been usd for film in many years. It is ISO in film just as it is in digital, and it means precisely the same thing.

11-22-2011, 11:26 AM   #5
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Thanks for the clarification Mark, I couldn't remember if film now used ASA or ISO.

However, what I was really questioning was whether there was some standards agency that oversaw the quoted manufacturer ISO values in the digital world, so that, for example, when comparing noise at ISO 1600 among different cameras, one could be confident that the quoted 1600's were consistent among the manufacturers.

From the articles by DXO labs that RAWR posted (haven't read them completely), it would appear that they are that standards agency, and their tests would indicate possible significant differences between what they find for ISO#, and what is stated by the manufacturer.

Not totally suprising as it would be good for marketing to quote a low noise at a high ISO#.

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11-22-2011, 11:43 AM   #6
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I don't think DXO oversees it; they just test

And was there a body that checked DIN/ASA/ISO of Kodak, Ilford, Agfa films in the film days to verify that they actually were what they said to be? It might be but I'm curious (and to be honest, I doubt it).
11-22-2011, 01:23 PM   #7
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It's a similar situation with ski bindings, where the "DIN" release setting is overseen by a standards agency in Germany (I think), so that a release setting of 10 is a 10 is a 10 (pretty much) regardless of binding model or manufacturer.
11-23-2011, 02:50 PM   #8
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DxO Labs (www.dxo.com) are merely a reputable French developer of image testing and other image processing software, who as a popular byline also do lens and camera testing (DxOMark - DxOMark by DxO Labs).

The standards for measuring ISO sensitivity in digital cameras (eg ISO 12232: 2006 - Photography -- Digital still cameras -- Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index) are set by the International Organization for Standardization.

ISO standards are voluntary agreements, but they can form the basis of mandatory national or industrial standards overseen by various regulators.

11-24-2011, 12:00 AM   #9
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The same but different

Re an earlier comment - The ISO measuring method parallels the ASA method so the two terms are effectively interchangeable.

BFN
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11-24-2011, 10:54 AM   #10
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QuoteOriginally posted by PENFRED Quote
However, what I was really questioning was whether there was some standards agency that oversaw the quoted manufacturer ISO values in the digital world
ISO = International Organization for Standardization :-). That is, ISO is actually the name of the agency, not the name of the particular standard for exposure index. It's only in the photographic world that we associate ISO with this particular standard, but pretty much every technical endeavor is standardized by ISO, and the exposure standard (formally, ISO 12232:2006) is but one of thousands of standards they administer.

QuoteQuote:
From the articles by DXO labs that RAWR posted (haven't read them completely), it would appear that they are that standards agency
No, they are just an indepndent "watchdog". ISO has way too many standards to be in the business of actively checking compliance to them all. But DXO doesn't seem to actually be checking compliance to the ISO standard itself, which, as defined, apparently allows for some lattitude, and hence variation between cameras. So two cameras might conform to the standard and atill yield slightly different exposures. That's what DXO is really meausring.
11-24-2011, 12:40 PM   #11
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So, would you be comfortable comparing noise levels across cameras based on nominal manufacturer ISO values, or do you think it's a case of caveat emptor, and results should be taken with a grain of salt?
11-24-2011, 01:16 PM   #12
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QuoteOriginally posted by PENFRED Quote
So, would you be comfortable comparing noise levels across cameras based on nominal manufacturer ISO values, or do you think it's a case of caveat emptor, and results should be taken with a grain of salt?
All specifications have tolerances. I haven't researched photo sensitivity standards, so I don't know if the tolerance here is 1% or 5% or 20% or what. Just for ducks, let's say the tolerance is 2%. Noise comparisons with a 2% difference would be within margin-of-error for many measuring systems -- too trivial to notice.

Another thing: digital sensors produce an output at their base ISO (sensitivity), which is boosted with analog amplifiers to reach higher ISOs. I'll guess that the amplification is within 1% tolerance -- that's how I would design those circuits. We're still at a trivial variance level here.

Also keep in mind that photo exposure levels are NOT linear, never have been. We don't increase shutter speeds as 1-1/2-1/4-1/8-1/16-1/32-1/64-1/128-1/256-etc seconds. We round-off those numbers, and the rounding tolerance is around 2%. Even greater tolerance would be acceptable. I dare you to find a noticeable difference between two shots, one taken at 1/50 second, the other at 1/60 second (around 20% difference). Such small variances just don't matter.

In other words: Don't sweat it.

Last edited by RioRico; 11-26-2011 at 11:40 AM.
11-25-2011, 11:05 AM   #13
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Good points all - must keep things in perspective. Even if a manufacturer's 1600 was really a 1500, that's still only a 7% difference.
11-29-2011, 05:49 AM   #14
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Are you basically asking, will camera a, b and c all have the same amount of noise at 1600 iso? If that is the question then the answer is no. Back in my prosumer days I looked into it a bit and I saw camera reviews and comparisons that would acutally give an equivalent for iso such as this cameras 200 iso is comparible to this cameras 400 iso for noise. I want to upgrade my body (I'm using an older ist-ds) specifically for better high iso performance. In other words, my camera has pretty lousy pictures noise wise at iso 1600 while newer bodys do not. From what I gather, iso may be somewhat standardized for exposure (1600 iso at 1/60 sec at f2.8 will give similar exposure) but they will not nesasarilly give the same amount of noise. Would you really expect a 100$ camera with a 1/2.5" sensor to have the same amount of noise at iso 1600 as a full frame dslr? I'm not sure how close current dslr's compare but I can tell you older and newer dslr's are not comparible and older prosumer and point and shoot cameras of the same era do not directly compare for noise.

If you dig through some of the reviews on dpreview, you will probably stumble on some iso performance comparisons that show two cameras with significantly diffrent noise levels at the same iso (and generally a way to compare them such as iso ??? is comparable to iso??? on these two cameras.
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