Digital BW, The Print

> From: Truman Prevatt [mailto:tprevatt@...]
>
> So there are two questions here - the one is technical the other is one
> of based ot the expectations of the individual. But you are right I've
> never seen a full up review of the digital backs like the ones you see
> on DPreview of DSLR's and prosumer digitals. On the other hand at 20
> grand a pop there is probably not a lot of demand either to justify the
> expense of the review.
>
> Clearly at some point the nosie is so low that it is not noticable.
> Currently we may not know that threshold.  In reconnoissance systems
> there are "rules of thumb" but they are based on the ability of
> automatic target recognition systems to perform rather than to please
> the eye and the eye can detect a lot of noise in images that these
> target  recognizes work just fine.
>
> I would be interesting to know these numbers. Otherwise the digital
> world is going to be based the hype of big pixel numbers and big S/N
> numbers.

I actually ran some rigorous tests on my Minolta DiMage 7. Since its raw
output is uncompressed, I was able to write a program that rounded it off to
fewer than the 12 bits that the camera produces. On a test shot of blue sky,
I found that I had to reduce to 8 bits in order to see any difference after
applying a drastic curve, and even then I couldn't see any actual banding.
My conclusion was that with that noisy CCD, the extra four bits of raw mode
data were complete rubbish.

My Canon 10D has nearly two bits better S/N, based on comparing results at
various ISO settings. This means that a 10-bit converter would be enough,
although 12 bits isn't a significant extra expense. I've never used a
scanning back, or seen any tested, but I see no reason why they couldn't
perform as well as a good CCD film scanner, which is probably around 12
bits.

Of course, the only reasons for needing more S/N is so that you can shoot in
very low light, or shoot very contrasty subjects and pull detail out of the
muck. I suspect scanning backs are intended to be used in the studio, where
one has control over the lighting, so don't even really need an enormous
dynamic range. Their primary design goal is just to get huge numbers of
pixels. The _really_ quiet sensors are used in astrophotography.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@...