[filmscanners] Re: Density vs Dynamic range

["" <peter@softwell.co.uk>]

>
> CCDs, in and of them selves, don't have anything to do with number of
> bits,
> as they are analog devices, but their dynamic range does...  If the CCD
> has
> a dynamic range of 5000:1, it will require a 13 bit A/D to be able to
> extract the full dynamic range of the CCD.
>
> Regards,
>
> Austin

Austin, I have kept out of this up to now, but I have step in to disagree
with this as it's just plain wrong.

The range of the CCD can be represented by any number of bits.  What will
vary is the degree of resolution of different values within the range. If
all bits off represents total blackness and all bits on represents clear
film (or no film) then the number of bits available defines the number of
steps within the range. The more bits available the smaller the steps and
the "smoother" the output.

The dynamic range of the scanner is determined by how much noise is
generated in the CCD.  It's relatively easy (in electronic terms) to
adjust the high point (all bits on) to be just at the clear film position,
but all electronic devices generate noise and there is no point setting
the bottom point deep in the noisy region.  Some manufacturers do this
which is why people complain of noisy shadows (slides) or skies
(negatives). (There are techniques of using multiple scans to eliminate
random noise, but they take time.)

Peter, Nr Clonakilty, Co Cork, Ireland

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