Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Jeff Medkeff <medkeff@g...> 
wrote:
> 
> 
> Paul D. DeRocco wrote:
> 
> 
> >>Dynamic
> >>range is just
> >>a ratio -- no more.
> 
> > Sure, but the dark end isn't limited by the numeric range--that goes to
> > zero, and you can't divide by zero. It's limited by the noise level, which
> > means that photographing the inside of your lens cap will give you nonzero
> > output. The average of that noise defines your black level.
> 
> Hey, from this statement it just dawned on me that one of the problems I 
> have had in understanding Roy's point is that he's been assuming that 
> you will always have noise in a pixel.

Yes, I think you always have some noise.  No matter how accurate anything is
there are always quantum mechanics effects.  In relationship to dynamic range
I'm only talking about the noise at the black (no signal) level.

> 
> With science cameras with highly reproducible bias, and dark current on 
> the order of one electron per six minutes (tenth of an hour) per pixel 
> or less, we take images all the time with nonspurious zero value pixels 
> (meaning very often zero electrons, and always less on average than 
> (e/ADU)/2 electrons) after bias subtraction. So the presence of 
> intrinsic noise in an image was not an a priori assumption on my part.

Obviously you have an extremely large dynamic range here.  The noise may be
only 1 electron but it's there.

> 
> Even with a Canon 10D at 0.5 ADU/sec/pixel, if you are shooting outside 
> during the day at 1/500 you are going to have a lot of noise-free 
> pixels. Of course here the offset is up around 110 to 130, and it isn't 
> as consistent, so bias subtraction doesn't work as well. It seems like 
> you might always have a noisy image come out of such a camera.
> 
> Maybe the noisy DSLR image is a good assumption on Roy's part now that I 
> think about it - at least for cameras currently on the market. But the 
> images can still be noise free at the analog level, and I was getting 
> tripped up there because, as Paul says, zero *is* a possible answer to 
> the question of how many electrons are in the well. Sorry if this has 
> contributed to any confusion.

Zero can be an answer to how many electrons are in the well, but that isn't
the denominator.  The denominator is the minimum signal that will produce
more electrons than no signal produces (i.e. noise).  In your science camera
example you have noise at 0 or 1 electrons, well the smallest signal needed to
produce 2 electrons is what goes into the denominator for dyn range.  It's
a subtle distinction but key to the definition of dyn  range.

Roy

> 
> --
> Jeff Medkeff
> Eagle River, Alaska