Digital Camera's Native ISO

[Guillermo Luijk]

Is it that the actual data of the raw files shot at different isos is the same, the MFDB only records the iso data I set as metadata, then the iso data is applied to the raw file automatically when the computer program opens the file - unless as in the case of C1 the program has not been taught how to do that?

If this were the case (a big if) then could I stop wasting my time by adjustin iso of raw files in an ignorant attempt to bracket my exposures and just do this in post, as adjusting iso would make no difference to the actual data captured by the camera (apart from the camera settings info. stored in the metadata)?

I would answer yes to both questions, but to make sure just open a couple of your DNG files (same aperture/shutter, different ISO) into Rawnalyze. If they have the same histogram, they hold identical RAW data and you are wasting your time with your ISO bracketings.

Regards

[BJL]

Are you using the same f-stop and shutter speed, different ISO speed setting and then getting same raw histogram with the raw converter that does not support that back's raw format?

If so, this sounds like the situation I have heard of with some (older?) DMF backs that the ISO setting has no effect on analog amplification and thus on the raw digital output, but just sets a flag in the raw file which tells the raw converter how much to adjust levels in the digital domain. The thinking being AFAIK, that the ADCs used in those DMF backs could handle the whole dynamic range from full well signal down to the noise floor anyway (being about 12 stops) so that variable analogue gain would be pointless, and indeed a source of degeneration of the signal.

[lowep]

Thanks Guillermo & BJL... this is quite intriguing. Maybe my Sinarback Emotion75 is a good solid "100iso"-equivalent (or maybe instead 200iso?) MFDB with a few extra buttons for amusement?

As you suggest Guillermo I downloaded the program Raw Image Analysis version 2.10.1.0 and installed it on my Win7 PC to compare 2 .dng files from the MFDB. Unfortunately the program could not fully open the files instead reporting invalid image file directory form (maybe as the Sinarback .dng format was not around when the the program was developed?) so no histogram :-( but the Rawanalyze program did provide the following information:

for image 1:
Sinarback Emotion 75, 2011:09:27 08:27:35, 4992x6668pix
ISO 100, F/5.6, 16666/1000000s, Corr 1/3 EV

for image 2:
Sinarback Emotion 75, 2011:09:27 08:27:43, 4992x6668pix
ISO 200, F/5.6, 16666/1000000s, Corr +2/3 EV

As you can see it reports 2 different values for the EV correction (my emphasis). Could this be a clue to how the .iso works?
 

[ejmartin]

Since your back outputs dng files, you have a couple of options.  Among them are

1) Download the command line tool dcraw and execute the command

dcraw -4 -T -D sinarfile.dng

for two images shot at different ISO.  This will process the raw file without demosaic or any other processing to a tiff file.  Open the resulting sinarfile.tif's and see if they have the same brightness/histogram; if they do then ISO is purely metadata on your camera.

2) RawTherapee 4.0 opens .dng files; it may be able to open yours.  There is an option in the histogram palette to display the raw histogram, so you should be able to see whether the histograms for two different images shot at different ISO.

Guillermo's histogrammar program may be a third option.

[lowep]

Aliens 4 - Humans 0  

* Reansalyse cannot open the Sinarback eMotion75 Raw .dng file

* DCRAW just flashes in a black box on my screen and disappears forever into the ether

* DCRAW X (for Mac) cannot open the eMotion75 raw .dng files

* Another nice raw interface program MEGUI that uses DCRAW can open raw .dng files from Canon 5D that I have on my computer but crashes when I feed it a raw .dng file from the Sinarback

So maybe my MFDB is the alien???

Meanwhile, I have been informed by a friend who knows about such things that it is true, the .iso switch on the Sinarback eMotion75 is mainly decorative, though have not been able to verify this myself yet.

If this is true, which increasingly seems to be the case, then what are the implications:

a) perhaps things do not always work as we are lead to believe they do
b) maybe this applies to your MFDB too?
c) perhaps more recent MFDBs work differently in this respect?
d) what is the point of relying on the histogram on the MFDB for such entertainments as ETTR etc if it is just hooked up to an iso switch that is mainly decorative….
e) am I the only one who didn't know this already???

Fortunately none of this need prevent me from continuing as I have always done opening the .dng files in C1 and boosting the exposure to the desired level. In my opinion This still produces a "better" 16-bit TIFF file than importing and exporting the .dng file from Sinar's own Captureshop or Adobe Raw (even though both of the latter two programs do recognise and automatically adjust the image and histogram to the iso setting in the metadata of the file) so life could be a lot worse.

[JonathanRimmel]

First off, thanks to everyone for your responses. It would seem once again Canon deviates from everyone else in how they handle ISOs. I use a Canon 7D at work and a Nikon at home. I am still a bit uncertain here though, what ISOs are considered "native" and additionally, do I understand correctly that it would be better to underexpose (using a fast enough shutter speed to prevent blur) rather than use ISOs above 1600 on any DSLR and then brighten up the image in ACR / Lightroom?

[ejmartin]

First off, thanks to everyone for your responses. It would seem once again Canon deviates from everyone else in how they handle ISOs. I use a Canon 7D at work and a Nikon at home. I am still a bit uncertain here though, what ISOs are considered "native" and additionally, do I understand correctly that it would be better to underexpose (using a fast enough shutter speed to prevent blur) rather than use ISOs above 1600 on any DSLR and then brighten up the image in ACR / Lightroom?

The ISO's that are purely analog gain on the 7D are 100-200-400-800-1600-3200-6400; all others are obtained by digital multiplication from the nearest of these (eg 160 is 200 pulled 1/3 stop, 250 is 200 pushed 1/3 stop).

As for underexposure at high ISO, yes there is not much advantage in going above 1600 as far as the quality of the raw data is concerned.  Though ACR/LR is not the most accurate processor if you need large amounts of exposure compensation; note also that ACR changes its internal defaults as a function of ISO, so the result out of the converter will differ from a file taken at a higher ISO (for instance more NR is applied to the higher ISO image).  You can be confident however that the raw files are equivalent and equivalent results can be had if the converter is up to the task.

[Guillermo Luijk]

d) what is the point of relying on the histogram on the MFDB for such entertainments as ETTR etc if it is just hooked up to an iso switch that is mainly decorative…

ETTR based on the histogram in a camera where ISO is pure metadata, is a guarantee to be underexposing by at least as many stops as the difference between camera's base ISO and the ISO set by the user. In other words, a ruin.

[lowep]

Apologies Jonathan for deviating so far into the bushes from the main fairway of this thread that is digital camera's native iso. To avoid this I have posted a new thread on MFDB iso & ETTR here. Hope this is ok.

[Graystar]

ETTR based on the histogram in a camera where ISO is pure metadata, is a guarantee to be underexposing by at least as many stops as the difference between camera's base ISO and the ISO set by the user. In other words, a ruin.

This is true of any camera.  To raise ISO is to gather less light (normally...if you're just increasing ISO to push the histogram then photometric exposure remains the same.)  It's just a question of when the exposure compensation gets applied...either immediately by the sensor's amplifiers, or later in post.

[JonathanRimmel]

Hmm. Perhaps I am just dense, but I am not entirely sure I understand this just yet. (although I might be getting closer)

It sounds to me like it would be more beneficial for me to set the ISO to the base ISO (100?) and use manual exposure mode to get the shutter-speed and aperture I want even if it underexposes and just brighten up and increase exposure later on in post.

But if the above was the case, wouldn't this go against the ETTR line of thought? Wouldn't noise be just as bad as increasing the ISO?

I just want to get the best IQ I can in very low light situations. It wouldn't be so bad if most of my low light photography didn't involve people, in which case I could just use a slower shutter speed.

[Guillermo Luijk]

To raise ISO is to gather less light (normally...if you're just increasing ISO to push the histogram then photometric exposure remains the same.).

This is not the case we are speaking here. We mean pushing ISO because maximum aperture/slowest shutter allowed do not permit ETTR. In that case pushing ISO to get ETTR improves noise in most cameras; not in the back discussed here (since ISO is just metadata on it).

[Guillermo Luijk]

But if the above was the case, wouldn't this go against the ETTR line of thought? Wouldn't noise be just as bad as increasing the ISO?

When RAW exposure is still not maximum, ETTR can be achieved in 3 ways:
- Use slower shutter speed
- Use wider aperture
- Pushing ISO

In some cameras (the MFDB discussed here, cameras with the Sony sensor: K5, D7000,...) the third option doesn't work, and ETTR must be achieved only through maximizing the amount of collected photons. In other cameras (most of them) pushing ISO is a good idea to achieve ETTR is a good idea if we cannot get it by optical means.

Regards

[JonathanRimmel]

When RAW exposure is still not maximum, ETTR can be achieved in 3 ways:
- Use slower shutter speed
- Use wider aperture
- Pushing ISO

In some cameras (the MFDB discussed here, cameras with the Sony sensor: K5, D7000,...) the third option doesn't work, and ETTR must be achieved only through maximizing the amount of collected photons. In other cameras (most of them) pushing ISO is a good idea to achieve ETTR is a good idea if we cannot get it by optical means.

So it looks as if I still want to use ETTR even in low light situations even if this means pumping up the ISO. I often find myself in situations where this is the only choice anyway. The fastest lens I have at my disposal at work is an F/1.8 50mm but this often times is not the right focal length for the job so I have to use one of the two other lenses here which are usually set at 5.6. This means a slow shutter speed. But with people in the scene I try best to stay above 1/80. This usually means an ISO of 1600 - 3200. This isn't as much of a problem with my own setup at home, but at work...

[Graystar]

This is not the case we are speaking here. We mean pushing ISO because maximum aperture/slowest shutter allowed do not permit ETTR. In that case pushing ISO to get ETTR improves noise in most cameras; not in the back discussed here (since ISO is just metadata on it).

Okay.  It's just that I personally don't consider that to be ETTR simply because ETTR is about increasing the light collected.  If you're increasing ISO just to take advantage of some read-noise quirk of the camera then that's a different technique altogether (and one I feel is of dubious value...but to each, his own.)

[Guillermo Luijk]

Okay.  It's just that I personally don't consider that to be ETTR simply because ETTR is about increasing the light collected.  If you're increasing ISO just to take advantage of some read-noise quirk of the camera then that's a different technique altogether (and one I feel is of dubious value...but to each, his own.)

I extend your definition of ETTR to RAW exposure. After all ETTR stands for 'Expose [the histogram] to the right', does ISO affect the histogram?.

If you feel ISO is of dubious value to reduce noise, just compare what happens in a Canon camera when ISO is pushed (same aperture/shutter on both shots):

Canon 350D:





Another example of marginal noise improvement thanks to pushing ISO (same aperture/shutter on both shots):

Canon 5D (image on the right):





In some other cameras ISO is (nearly) useless to reduce noise (the K5 on the left would have hardly got any benefit from pushing ISO).

Regards

PS: the fantastic still life picture of mine is copyrighted, please don't make money out of it without written permission.

[Graystar]

If you feel ISO is of dubious value to reduce noise, just compare what happens in a Canon camera when ISO is pushed (same aperture/shutter on both shots):

But when would you ETTR by 4 stops?  If every element in your scene is either 12.7% gray or darker, the most you can overexpose by is 3 stops.  I find that saturated reds and blues really limit ETTR.  Taking a pic of a red fire truck?  You'll get 2/3rds of a stop before you clip the red channel.

I was wondering what improvements you'd get with only 2 stops of ETTR.  I don't have a Canon 5DMII, but I figured I could fake it.  I downloaded the ISO 100 and ISO 400 test images from Imaging Resource, increased exposure on the ISO 100 image, and then compared the three darkest gray patches from the ISO 100 image to the gray patches that were the closes match in the ISO 400 image.  Then I did the reverse (though that didn't quite work as well.)  The results are in the attached image...

As we already know, applying EC to a very dark tone results in more noise than having set a higher ISO.  That's visible in the first comparison.  But with an increase of two stops, the difference isn't much.

However, when ETTR is used, exposure isn't increased in post...it's decreased.  So the second comparison shows the ISO 400 image decreased (I should have played with EC more to better match the patches...sorry.)  Even if you zoom in to 300%, there's no real discernable difference in the noise levels.

This is the basis of my feeling that boosting ISO is of dubious value...you'll rarely boost more than two stops, and when you reduce exposure in post the apparent noise appears the same.  Just wanted to clear that up.

PS: the fantastic still life picture of mine is copyrighted, please don't make money out of it without written permission.

I'll try to fight the urge 

[Guillermo Luijk]

I was wondering what improvements you'd get with only 2 stops of ETTR.

That strongly depends on the sensor technology and how far from saturation we are exposing (i.e. how deep the shadows are). For Canon cameras, in the deep shadows going from ISO100 to ISO200 nearly halves the visible noise (6dB of improvement), and pushing to ISO400 improves by some extra 4dB. So the 2 stops from ISO100 to ISO400 mean an improvement of up to 10dB, i.e. 3 times less noise in the deep shadows for 2 extra ISO stops.

Following the thick black curves you get the SNR improvement for these 3 cameras:




As can be seen, most of the improvement is obtained in the first ISO pushes (going from ISO100 to ISO200 improves more than going from ISO200 to ISO400, and so forth). Improvement in Nikon sensors is more reduced than in Canons. Improvement in the K5 (Sony isoless sensor) is negligible.

Regards

[Graystar]

As can be seen, most of the improvement is obtained in the first ISO pushes (going from ISO100 to ISO200 improves more than going from ISO200 to ISO400, and so forth).

Yes, I understand the numbers...I do believe that the read-noise issue is real.  I simply question whether it's visible (in a pixel-peeping sort of way, mind you) in actual photographic works.

[BJL]

An obvious reason why, with many cameras, it makes sense to increase ISO above the minimum (which I insist is not the ”native" ISO, and has nothing to do with a film's ISO speed) when you cannot get enough light to fill the photosites is to reduce the effect of noise sources that enter after the amplification but before conversion to digital is completed. One such source is quantization noise in the ADC, when its dynamic range (true bit depth) is less than that of the sensor's signal. Another is read noise entering during signal transmission after the amplification. The latter is a factor in CMOS sensors that do the amplification in charge transfer from photosite to the edge of the sensor, before transmission along the sensor edge and so on. At least some Canon sensors described in research papers do the variable gain this early; I do not know for sure about any sensors actually in production but the graphs above suggest that Canon does this more than others.

As to the debate about ETTR, I think of this strategy as exposing the ADC to the right, ensuring that the most significant bits in the output are used.

P.S. If using the alternative is using a lower ISO setting and then doing extra work pushing levels up in post, and the results from that extra effort are at best no better and more likely at least a little bit worse, what reason is there for not using the higher ISO setting that gets the midtones at the normal, convenient level?