RAW histograms from 30D - interesting combing patterns

Started Jun 28, 2006 | Discussions thread
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Ruevs Forum Member • Posts: 69
RAW histograms from 30D - interesting combing patterns

Hi all,

So I have finally turned into an equipment measurbator ( http://www.kenrockwell.com/tech/7.htm ) at least for a couple of days - but at least this is the ideal forum to share my illness If you do not like technical discussions now is the time to press the back button.

It has been a month and a half since I bought a 30D (after 14 years with two film SLRs) and I really like it. However the programmer in me could not help but thinker with all the (mega) bits coming out of this wonderful camera. I have been following the forum for quite a while and recently read John Sheehy's (I hope you are reading this) threads on the 30D intermediate ISO behaviour with great interest. And I have known about Iris and dcraw for quite a while and used them with a few RAW files I downloaded. Well now I have my own RAW files to tinker with.

I was not happy with the fact that there is no good way to display RAW histograms in their full 4096 bin resolution. So I wrote my own program to do it. You can read all about it and download it here:
http://www.pages.drexel.edu/~par24/rawhistogram/rawhistogram.html

(For those who may complain - the program is absolutely free with source code - nothing commercial at all)

The results were quite interesting to say the least... These are histograms of boring pictures of the sky (the forum does not show PNG images so please click the links):

ISO 1600, EC 0, 1/200 f4.5, flash, FEC 0

ISO 100, EC 0, 1/13 f29

So far so good... however a few hundred frames later when shooting a wedding (yet another my first wedding thread:-) this happens:

ISO 100, EC 0, 1/400 f2

You can see the combing artifacts in the histogram - this is definitely not "real RAW" data as it came off the A/D converters. Every 27 levels there is a "gap" - i.e. there are no (0) pixels with that brightness.

And again later on in completely different conditions, but now the gaps are every 18 values:

ISO 800, EC +1/3, 1/250 f1.8, flash, FEC 0

And so on for all of the photos I looked at - the same combing patterns in the histograms with varying steps. The bottom line is that the 30D seems to manipulate the RAW data coming from the sensor (A/D converters really) before writing the CR2 files. But it does not always do it. I find this very interesting, unfortunately right now the camera is tied up in a 4 week time lapse movie so I can not move it - and therefore can not take more controlled test shots.

I would really like to hear what you think about this.

Best regards,
Peter

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