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10-19-2010, 07:18 AM   #1
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How ISO 3200+ works on your K-5 [technical]

This is my understanding of how the K5 handles high ISO. If I am mistaken, please correct me!!!

Ok, so first of all, your K-5’s highest “native” ISO is 1600. That’s why ISO 1600 is the highest selectable ISO in bulb mode. All higher sensitivities are done with smoke and mirrors, I’m afraid!

The interesting thing about this is your RAW files aren’t really RAW!

Here’s how it works (in slightly simplified language):

At ISO 1600 and lower, when you press the shutter button the Prime II processor sends a request to the sensor for a read-out of all the pixel locations. On previous generations of sensors, this information was sent to the processor in analogue form to be processed into digital. However, on this new sensor things are done a bit differently. The sensor gathers analogue data from the photosites. This information is immediately converted to digital data (1’s and 0’s) on the sensor. Please note that no new information is added to the image from this point on. The digital data is then sent to the Prime II for processing into Jpegs or demosaic’d (I think?) and sent straight to the card in the case of RAW.

To explain what happens above ISO 1600 I will use the example of shooting at ISO 3200. Because the sensor is incapable of capturing images any faster, the camera does some tricks to get this “higher sensitivity”. In this case, when you press the shutter, the Prime II asks the sensor to take a readout at -1EV. In other words, the image is underexposed. This under-exposed analogue image is immediately converted to digital on the sensor and sent to the Prime II for processing. This is where it gets interesting when shooting in RAW.

Theoretically, RAW images are digital conversions of the analogue signal received by the sensor, with as little processing as possible, so that the user can do their own custom processing on their computer. However, when the K-5 is set to ISO3200 the digitized image from the sensor is actually underexposed by 1 stop! The Prime II then adds 1 stop (+1EV) to the image (,adds NR?) then sends the image to the card as a RAW file. This is a fair bit of processing for a “RAW” image! What we’re getting here is what you might call a “pseudo-RAW” file! So much for a RAW option which gives the user the power to process the image as they prefer!

The situation gets worse when you get to ISO 52100. By this stage, the sensor is supplying an image to the PRIME II that is 5 stops underexposed (-5EV)!!! This is then brightened by +5EV and (most likely) NR’d by the PRIME II before sending to the card as a RAW file. All this processing explains the wide variation of file sizes for RAW images. Generally, the higher the sensitivity, the larger the file size! Which basically means more noise information is being recorded.

In film terms, what the K-5 is essentially doing at ISO 52100 is push-processing ISO 1600 "film" by 5 stops in-camera!


The point being, you can get a more “true” RAW file at ISO 52100 by shooting at ISO 1600 at -5EV! And if you did, you wouldn’t have the reduced-buffer issue seen at higher in-camera ISO settings! The problem is, of course, reviewing the shots in-camera would be nearly impossible to do (especially accurately) because the images you’ve just captured are so under-exposed! I would expect the LCD would look almost black. Histograms would be useless of course.

In my view this calls for a feature request from Pentax in an upcoming firmware release: Can we please have “true” RAW image capture at high sensitivities? This would mean brightening the image with the PRIME II, but only for in-camera review (maybe include a jpg “thumbnail” that has been processed to correct EV only?). The RAW file actually stored on the card must be the under-exposed image, with as little processing as possible done to it. We will take it from there, thanks!

Now, if all the above is correct, we should be able to get results just as good from a RAW image shot at -5EV at ISO 1600 as we can get from a normally-exposed image shot at ISO 52100. Can someone with a K-5 please post one of each in RAW so that an NR whiz can do their thing? Thanks!


And thanks for reading. I'll be interested to see the responses I get.


Last edited by secateurs; 10-19-2010 at 07:23 AM. Reason: small corrections and additions
10-19-2010, 07:33 AM   #2
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Evidence is important. Where are your sources?

This is the first I've heard of it, I'm going to do some searching later though.

Last edited by Daemos; 10-19-2010 at 07:38 AM.
10-19-2010, 08:07 AM   #3
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Hi Daemos. Thanks for your post.

I have collected this information from various sources, including various web articles, observations from real-world users in the PentaxForums, and my own knowledge of electronics. I quickly banged this out before going to bed (now i can't sleep!!!), so i didn't bother citing all the sources. Some of it is speculation.

If you have queries on sources for particular information, please ask and i'll try to dig them up for you.
10-19-2010, 08:08 AM   #4
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As I understod things the sensor does the multiplication of EV instead of the Prime II and that the Prime II only kicked in at iso 25600 and 51200, thereof the therm Extended Iso.

(so we should see the same mecanism on the D700, A55 and A580)

10-19-2010, 08:13 AM   #5
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Secateurs, your points are 100% speculative.

Falconeye and others with more expertise would be best to discuss this. You might want to review falconeye's blog for his technical discussions on noise etc.

In the meantime, can you fit your discussion around diagrams like these? They reveal the complexity of assessing NR within the signal processing system of a camera like the K-5. The diagrams relate to Sony sensors and systems, but it's a Sony sensor in the K-5:




Exmore sensor workings...
10-19-2010, 08:38 AM   #6
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QuoteOriginally posted by secateurs Quote
All this processing explains the wide variation of file sizes for RAW images. Generally, the higher the sensitivity, the larger the file size! Which basically means more noise information is being recorded.
Is this data true? I only have a K10D but others here have the K5 now. Does the RAW file size stored on the SD card change based on the ISO above 1600??

I realize the diagram posted by Rawr (originally from Falk as I recall) is a graphical representation but in the top Exmor graphic all the digital data are equal sized whereas the noise glyphs change in quantity from left to right.

Again, does the RAW file size change based on ISO?

Edit: Ok I just saw dgais response in the K5 Buffer thread. He has posted the file size data. Interesting, it does change!

Last edited by JackBak; 10-19-2010 at 08:55 AM. Reason: Further Comment
10-19-2010, 08:38 AM   #7
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In the above scenario the critical question is whether anything irreversible is done before saving the pixels as a raw file. The push up operation in 1 EV steps (multiplication by 2) in itself shouldn't lose any information, or, in other words is fully (and trivially) reversible. Noise reduction, for example, would result in loss of information, and (therefore) be irreversible.
10-19-2010, 08:57 AM   #8
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Unless you have definitive evidence, all that you have stated is speculative conjecture. You have unwittingly just added to the body of pseudo science being passed off as fact that is so rampant on this and other forums of late. I would expect some other armchair "expert" to come along and quote your unproven hypothesis and perpetuate the pseudo science. After all, if it gets repeated long enough it must be true...

10-19-2010, 09:10 AM   #9
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Well i am sure that there are some more spezialised guysfor this thread, but in fact i do not really see there any problem with this "ISO workaround" i don't know anything about a sensor capable of ISO 102000 like the D3x offers, and even older Sensors where "capable" of ISO 3200 oder 6400, i think this was done in almost the sameway.

When you want a -5EV underexposed shot, just use the EV-correction and ISO 1600
10-19-2010, 09:36 AM   #10
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QuoteOriginally posted by creampuff Quote
Unless you have definitive evidence, all that you have stated is speculative conjecture. You have unwittingly just added to the body of pseudo science being passed off as fact that is so rampant on this and other forums of late. I would expect some other armchair "expert" to come along and quote your unproven hypothesis and perpetuate the pseudo science. After all, if it gets repeated long enough it must be true...
I agree with Creampuff. Nothing you've said is substantial. Pentax has been quite clear, despite some mix-ups in communication that the standard ISO range is 100-12,800, and that 80, 25600 and 51200 are "expanded" software tweaks. (Sensors only have one "Native" ISO, which is 100 in the case of the K-5. The rest of the ISO's are by increasing the gain of he sensor, or digitally pushing the sensor) So unless you're saying that they are just straight up lying to us...

My guess would be that since, to achieve higher ISO from the native ISO of 100, the gain of the sensor is increased. Since both that and long exposures both increase sensor heat, I would guess that the ISO 1600 limitation in Bulb mode is probably do to unsafe heat levels above that.
10-19-2010, 09:40 AM   #11
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At the risk of making myself look like I have completely and utterly missed the point, if this theory is correct (and I suspect that something similar IS happening) then I should be able to do a similar experiment on my K10D by taking an under exposed image at ISO1600, then manually increasing the exposure on it and applying NR, and this should result in a better image than the native ISO1600 shot. Is that right?

Obviously the K10D uses an older sensor but would the methodology still apply?
10-19-2010, 09:44 AM   #12
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If I recall correctly, falconeye and gordonbgood showed that above 1600 ISO, one would not be able to tell the difference between analog and digital amplification on the older Pentax cameras, but they didn't prove it was one or the other.

Also, there was no source from Pentax or other manufacturers that explicitly state what "expanded" technically is. As far as we're concerned, it could simply be a marketing term. As the aforementioned have shown, for example, the K-x's native ISO is somewhere between ISO 100 and ISO 200, yet anything below 200 is "expanded"

Are the compressed RAW filesizes the same for ISO 1600 EV-1 and ISO 3200? ISO 1600 EV-2 and ISO 6400 when averaged over multiple shots of the same scene?

If it's just digitally multiplying and thus leaving gaps in intermediate bins, should they not be?

Last edited by Eruditass; 10-19-2010 at 09:57 AM.
10-19-2010, 09:45 AM   #13
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Howard2k, no, you would probably end up with even more noise.
10-19-2010, 09:52 AM   #14
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QuoteOriginally posted by pikoli Quote
Howard2k, no, you would probably end up with even more noise.
Thanks, I figured that would be the case, otherwise we would all be doing it already.
10-19-2010, 09:58 AM   #15
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Bonjour a tous.
Moi je suis pas SUR de ce que je parle (non verifie)
mais je pense que secateur a au moins partiellement raison.

pour plusieurs choses.
Si le capteur etait NATIF en 52100 Iso (par exemple) le mode Pause "Bulb" serait possible DANS ce mode, ce qui n'est Absolument pas le cas, il est fixe a 1600 Iso Max (Du a la surchauffe du capteur ??) mais une raison est la (Presente).

Par contre ceci va nettement dans le sens de la speculation de Secateur:
Voir Ici:
http://www.blog-couleur.com/?Qu-est-ce-que-la-sensibilite-ISO

Le lien est en Francais, desole.
Et j'attire particulierement votre attention sur cette partie ci:
"Cette sensibilité naturelle du capteur n’est pas modifiable. Elle est fixe et invariable. Il existe pourtant un moyen de simuler un changement de sensibilité qui consiste à amplifier la luminosité en sortie du capteur exactement comme l’amplification du son dans le domaine de l’audio. Il n’y a donc pas a proprement parler d’augmentation de la sensibilité, mais seulement une interpolation de la sensibilité existante. L’avantage de cette méthode est qu’elle conserve l’analogie avec la sensibilité ISO telle qu’elle est pratiquée en argentique, l’inconvénient est que chaque gain de sensibilité ISO correspond à une dilution de l’information."

Voila, sinceres salutations, Jpette
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