Peter Ruevski
New member
This post may sound quite cryptic unless you have read this http://www.openphotographyforums.com/forums/showthread.php?t=546 thread. I am sorry, but otherwise I will have to repeat many things.
Some time ago (see the above thread) I discovered that the (raw) histograms of some Canon 30D raw files had interesting combing patterns. The combing is an effect of scaling the raw data (i.e. multiplying it by a number or "stretching" it) before it is written to the raw file. This by itself was not a mystery. The real mystery was why is Canon doing this and under what circumstances. This was particularly curious in light of the fact that Canon cameras usually save real, untouched raw data to their raw files (this is not at all the case with most other DSLR brands).
The mystery remained until recently when I received an email from someone that had solved the puzzle.
It turns out the scaling is done at apertures larger (smaller f-number) than f/2.8 in order to compensate for the signal drop caused by the light rays hitting the sensor at oblique angles. Unlike film, digital sensors do not "like" oblique light angles since the micro-lenses in front of the pixels are not optimized for them. As a result for really wide apertures the signal (counts) given by the sensor is not as high as it should be (i.e. it is not proportional to the increase in light intensity) and the firmware is correcting for it by multiplying the raw data by a constant scaling factor.
I created a page to describe this in detail, you can find it here:
http://www.pages.drexel.edu/~par24/rawhistogram/CanonRawScaling/CanonRawScaling.html
If you want some more background look at these links:
http://forums.dpreview.com/forums/read.asp?forum=1019&message=19009766
http://www.pages.drexel.edu/~par24/rawhistogram/index.html
Some time ago (see the above thread) I discovered that the (raw) histograms of some Canon 30D raw files had interesting combing patterns. The combing is an effect of scaling the raw data (i.e. multiplying it by a number or "stretching" it) before it is written to the raw file. This by itself was not a mystery. The real mystery was why is Canon doing this and under what circumstances. This was particularly curious in light of the fact that Canon cameras usually save real, untouched raw data to their raw files (this is not at all the case with most other DSLR brands).
The mystery remained until recently when I received an email from someone that had solved the puzzle.
It turns out the scaling is done at apertures larger (smaller f-number) than f/2.8 in order to compensate for the signal drop caused by the light rays hitting the sensor at oblique angles. Unlike film, digital sensors do not "like" oblique light angles since the micro-lenses in front of the pixels are not optimized for them. As a result for really wide apertures the signal (counts) given by the sensor is not as high as it should be (i.e. it is not proportional to the increase in light intensity) and the firmware is correcting for it by multiplying the raw data by a constant scaling factor.
I created a page to describe this in detail, you can find it here:
http://www.pages.drexel.edu/~par24/rawhistogram/CanonRawScaling/CanonRawScaling.html
If you want some more background look at these links:
http://forums.dpreview.com/forums/read.asp?forum=1019&message=19009766
http://www.pages.drexel.edu/~par24/rawhistogram/index.html
