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Accuracy of ISO readings

Steppenwolf 9 1.2k
11 Dec 2012 1:44PM

But. But. If you take photos with two cameras at the same exposure (shutter speed and f-number), and leave the ISO up to the camera (auto ISO), you’re really comparing metering systems rather than ISO sensitivities. (LenShepherd suggested as much above.) One camera’s matrix meter might try to retain detail in a bright sky, for example, while the other camera favours the foreground. Wildly different ISO settings (and JPEG brightnesses) could be chosen accordingly.

If I take two identical pictures (same frame, same end result in terms of brightness) using the same shutter speed and aperture, I'd expect roughly the same ISO on both shots. There may be other factors involved (such as the transmission efficiency of the lens, shutter accuracy, aperture accuracy) but assuming these are all pretty similar I'd expect a similar ISO to be selected. Admittedly the metering systems may be using slightly different algorithms but the photos looked very similar.

However, the DXO tests presumably are measuring the native sensitivity of the sensor itself in some way. The fact that these tests show the OMD is actually using ISO800 when it says ISO1600 (according to DXO) needs quite a bit of explaining away IMO. Like I said, I've read in AP reviews that there are large differences between ISO calibrations on different makes of camera but I didn't realise it was anywhere near 1 stop.
Paul Morgan 19 19.5k 6 England
11 Dec 2012 4:10PM
A stop difference ?

I fail to see this using two different camera`s in a controlled studio environment using the same ISO`s, same shutter speeds and same apertures.
11 Dec 2012 9:33PM

Quote:If I take two identical pictures (same frame, same end result in terms of brightness) using the same shutter speed and aperture, I'd expect roughly the same ISO on both shots.

I would too (roughly, as you say), but only if shooting raw files and processing in a similar way. With JPEGs all bets are off, since there are advertised features like D-Lighting that can massively affect the brightness, and behind-the-scenes tone curves that act as ‘black boxes’ in cameras like the V1.

Still, if the two cameras take similar photos with identical exposures, does it matter if they report different ISO values? The reported value has no effect on noise, of course: that depends on the exposure and the technical merits of the sensor.

If you take a photo of a dull low-contrast scene (like a ceiling) with both cameras, with the cameras in fully manual exposure and shooting raw files, using the same shutter speed, f-number (preferably stopped down a bit to reduce vignetting), and ISO setting, how do the photos compare in brightness when processed with the same software?

Here’s DxOMark’s explanation of ISO sensitivity. It’s a saturation-based definition that says essentially nothing about the sensor’s inherent sensitivity, but which does allow standardised comparisons between different cameras.
User_Removed 10 4.6k 1 Scotland
12 Dec 2012 10:42AM


Still, if the two cameras take similar photos with identical exposures, does it matter if they report different ISO values?.

As I suggested above, Samuel, it doesn't matter tuppence to our use of the cameras.

But I think one concern expressed by others on this thread maybe does have some commercial validity. If, for example, Maker A calls a degree of signal amplification ISO 6400 and Maker B calls the same degree of signal amplification ISO 12800 and then Maker B (or a reviewer) claims that their camera has the same IQ at ISO 128000 as Maker A's comparable model has at ISO 6400, then that may be seen as "cheating".
thewilliam 12 6.1k
12 Dec 2012 12:22PM
The difference will be obvious when the camera is set on manual and an external meter has been used to determine exposure. The picture will only be correctly exposed if the camera's ISO setting is correct.

So it does matter!
Sooty_1 10 1.5k 221 United Kingdom
12 Dec 2012 2:55PM
I thought the whole point was that it was a standard? That's what ISO is, the International Standards Organisation. If manufacturers are misrepresenting the sensitivity and amplification levels of their sensors, then there is a problem. They are relying on the vast majority of camera owners not comparing different makes, but tbh, it makes little difference in the real world. Variations in exposure of a stop can easily be adjusted, and most of it is subjective anyway. There isn't a "right" exposure, just the exposure you consider "right".

What is true, is that exposure values are measurable and standard, so it shouldn't be beyond the wit of man to use something (say 18% grey) as a standard by which ISO can be actually standardised and manufacturers will have to abide by the figures.

thewilliam 12 6.1k
12 Dec 2012 8:03PM
When I did a zone system workshop, the instructor's first words were to be skeptical about box ISO values. The only way to know that it's right is to do a test!
EsaT 7 21 Finland
18 Dec 2012 4:49PM

Quote:However, the DXO tests presumably are measuring the native sensitivity of the sensor itself in some way.
Photosites of sensors don't have any native absolute sensitivity...
Only some arbitrary ratio for what signal they give out for certain amount of received photons.
Which is of zero use for photography until that signal is processed and "calibrated" to standard.

And that photographical ISO standard is all about producing correctly exposed image for particular combination of f-ratio/shutter speed/ISO for certain illumination of scene.
That same processing and calibrating was done even with film, it was just called as developing process at the time.

Why ISO isn't ISO
Steppenwolf 9 1.2k
19 Dec 2012 9:42AM
If DXO really do check ISO by measuring "the amount of light it takes to saturate the sensor to pure white" then this seems pretty silly. They would just be measuring the "capacity" of the photosite to collect photons - which seems to me is more related to the dynamic range of the sensor than the ISO capability. I'm not sure who "Ctein" is but what he says doesn't make a lot of sense to me. When I read something on the internet that doesn't add up I tend not to believe it.
mikehit 11 8.0k 13 United Kingdom
19 Dec 2012 9:54AM
This explains their reasoning. It seems they are not measuring the capacity but responsiveness:

Quote: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...
As tests show, the ISO settings reported by camera manufacturers can differ significantly from measured ISO in RAW. This difference stems from design choices, in particular the choice to keep some “headroom” to avoid saturation in the higher exposures to make it possible to recover from blown highlights.

Steppenwolf 9 1.2k
19 Dec 2012 10:44AM
"The first relates sensitivity to the exposure necessary to saturate the camera."

That seems to be saying that DXO do measure the amount of light needed to saturate the sensor. That seems strange to me as I'd have thought that was more related to dynamic range than ISO. ??

In any case my rough and ready attempts to compare ISO just involved taking two identical pictures and seeing what the recorded ISO readings were. Considering that both the sensors of the V1 and A77 are made by Sony (I'm pretty sure the V1 is a Sony anyway), I'd guess this should be a reasonable test.
mikehit 11 8.0k 13 United Kingdom
19 Dec 2012 11:21AM
But isn't that how film ISO was determined? From that link:

Quote:To be easily understood by photographers, the ISO sensitivity of digital cameras has been defined such that it is similar to the ISO sensitivity of photographic film cameras, thus lower sensitivities require longer exposure for the same luminance to produce the same result.

Without precise details on their methodology it seems to me that you are talking about the same thing: the only thing you are differing in is whether you measure a mid-point signal with the image as a surrogate marker (your measurement) or select an absolute indicator (DxO technique). And they are using the ISO defined endpoint (saturation).
And I do not know what method the manufaturers use - if it is the same as DxO then they need to answer a few questions.

This is how I see their (DxO) testing: With a film you can expose the film and the negative will have a certain brightness and you can translate that to a print by standardising the processing of the negative/paper. With digital, whatever the signal that comes out of the sensor you can adjust the final image brightness in the onboard software. In other words, two manufacturers could use exactly the same sensor but have different RAW outputs depending on how the signals from the photosites are converted to image pixels. In other words you are measuring the combination of sensor in combination with other technology (film analogy: give a film negative to two different labs. Is any difference in brightness of final image down to the film used or the way it was processed?)
So the only consistent factor is the electrical output of the sensor (photosites) and they measure exposure and the signal output and see how quickly it saturates. It is then up to the manufacturer how they best harness that technology.

I agree that the same test could also looks as though it is measuring dynamic range, though that would be the difference between noise level and photosite signal.
Steppenwolf 9 1.2k
19 Dec 2012 1:34PM
Maybe, but consider an example: imagine a perfect sensor that has 100% efficient pixels that basically count the photons that are hitting them. Imagine that they could produce two versions of this sensor, one that maxed out at 128 photons and another at 256 photons - I imagine this is possible. Sensor 2 would have better dynamic range but the ISO ratings of both would be the same (I think). Yet using the "saturation" test the ISO rating of sensor 2 would appear to be half that of sensor 1. There's something here that doesn't add up.

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