Digital BW, The Print

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.

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.

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.

--
Jeff Medkeff
Eagle River, Alaska