Aahz:
| That reference is a book by Fil Hunter, Paul Fuqua, and Steven Biver
| (borrowed it from my local library, yours may have it, too). Quacking
| around for "camera sensor linearity", there are similar comments online:
[...]
| http://www.qsimaging.com/blog/?p=81.
This is actually a very interesting thing; instead of being a 'blog' as
such it appears to be essentially be a technical note from a maker of
specialized CCD cameras for scientific purposes. As such I will quote
two important bits:
Normalizing the signal level allows the linearity of the sensor
to be confirmed down to very low signal levels.
[...]
[...] Full frame, non-anti-blooming CCDs, like those in the QSI
604, 616 and 632 exhibit linear behavior across their entire
dynamic range.
This is for (presumably quite expensive) specialized scientific
cameras, not DSLRs, but I would be quite surprised if DSLRs were
substantially different from this. The underlying sensors they use
appear to be made by Kodak.
(Note that the 'full frame' that QSI is talking about here is not the
same thing as what photographers mean by the term.)
| http://harvestimaging.com/blog/?p=1154
I am not certain how to interpret these results, given that an
earlier entry in this series (http://harvestimaging.com/blog/?p=1140)
says that the data comes from a camera with a 10-bit ADC. Have such
DSLRs been made any time in the past half decade?
It is also worth reading the scale of the deviations reflected on these
graphs; the 'large swings' go to 0.75% deviations from the expected
results.
On the whole I have to conclude that there is not enough information
in this series of articles for me to understand the results it is
reporting. There is no discussion of the effects of read noise,
photon shot noise, how the 'gain' being discussed relates to terms I
understand, and so on. I assume that the author is writing for an
audience that doesn't need those discussions, but that means that
there is all sorts of background stuff here that I simply don't know.
Drawing any conclusions without knowing this background strikes me as
unwise.
|
http://kobus.ca/research/publications/02/camera_characterization/camera_characterization.pdf
This dates from 2001 or earlier. State of the art in DSLR ADCs has
changed drastically since 2001, to put it one way. I did not read
further once I noticed the date.
| http://www.dpreview.com/glossary/camera-system/sensor-linearity
dpreview is not a technical publications and, among other things,
measures camera 'dynamic range' based on JPEG results. I don't think
their articles can be given any particular weight without sources and
far more information.
| It makes sense if you think about it: the ADC is not going to be
| perfect, and it will be less perfect as you approach the ends of the
| dynamic range.
There are some ferociously technical people who hang around
dpreview.com's forums (they get into very low-level analysis of sensor
performance, including for astrophotography). I've never seen them
discuss any evidence that camera ADCs are at all non-linear in general
at either end of the dynamic range.
(Some cameras have pattern noise in the (deep) shadows but that's
another issue. And then you get into the fun general issue of photon
shot noise.)
I have read solidly technical people on dpreview.com who believe very
strongly that *colour response* is non-linear and may/will shift under
over or under exposure. My own view is that since almost no one has
noticed this shift, it cannot be very large and so I am not going to
worry about it for my own photography.
As I understand what these people have said, the shift in colour has
nothing to do with ADC linearity (or lack thereof) but instead involves
the specific spectral response curves of the Bayer filters and the
underlying sensor pixels.
- cks
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