Re: filmscanners: RE: filmscanners: VueScan clipping & flat images

["Rob Geraghty" <harper@wordweb.com>]

"Jawed Ashraf" <Jawed@cupidity.force9.co.uk> wrote:
[so much I can't begin to respond to much of it...]
> > adjusting levels actually does.  But presumably the fewer bits from the
> > A/D, and the smaller the input range, the poorer the result would be.
> I dare say you're correct.  If the scanner is, internally, 8-bit, then
some
> data values have gone "unused" in your example.

It was an artificial example, but I was trying to demonstrate how important
the bit depth of the A/D is, even when the film is a neg, because if the A/D
is set the return a fixed range of numbers based on a certain fixed voltage
range, you're going to lose a lot of the potential resolution of the A/D
when you scan colour negs.  If there's not enough bits in the A/D and the
density rang eof the neg is very narow, presumably there's a risk of
posterisation.

> BUT, because of exposure control in scanners, the scanner and
> its controlling software in your workstation *may* manipulate the
> exposure (brightness of the light or duration of exposure per scan
> step) and/or manipulate the gain of the electronics (like the volume
> control on your hifi).  So the end result is, frankly "there's no
predicting"!

But in practice I don't know of any consumer CCD scanner which does adjust
the gain of the input.  AFAIK the only thing which is done is to adjust the
exposure (integration) time.  Adjusting the integration time won't expand
the input range, although it may shift the input to a level further above
the noise floor.

> What happens when the image "naturally" contains more than 8 stops
> of dynamic range (how does the 8-bit scanner and associated software
> cope)?

And my point being what about when that 8 stop range is compressed into a
fraction of the possible values the CCD can read?  A normal colour neg is a
long way from the range of a slide in terms of the transmitted light.  A
slide like Provia 100F gets pretty close to the range from no transmission
(black) to total transmission (clear or white).  But a colour neg has its
sensitivity compressed into a much more narrow range.  If you don't have
enough bits, you won't get the subtle tones whichare in the film.

> The gamma function that is used to convert from scanner RGB space to a
> normal graphics datafile space (say, sRGB which is defined with an
in-built
> gamma factor of 2.2) has the side effect of "squeezing" the data in a
> non-linear way.

Real life is not linear after all. :)

> 8-bit data is basically working at the limits here

I merely used it as an example.  My LS30 has a 12 bit A/D although I can
only access 10 bits.

> So this experiment shows that on my monitor 6-bits are all that are needed
> to satisfy my perception.

Provided you can get the right 6 bits.  For instance, my experience with
Nikonscan has been that 8 bits are useless with a high contrast silver based
B&W neg.  It just doesn't handle the range of density.

> The LS40 and LS4000 (used with Nikon Scan) do.  It's what happens when the
> auto-exposure kicks in (I believe) which changes the brightness of the
> "lamp" (there's logic for why I could be wrong - I'll let somebody else
> argue the point).

No, they don't.  As Ed explained, they change the exposure time not the
brightness of the LEDs.

Hm.  Thunderstorm.  Must get offline!!

Rob