[filmscanners] RE: 12 vs 16-bit A/D conversion. Was: WinXP and DimageScan Dual II and more

["Austin Franklin" <austin@darkroom.com>]

Hi Arthur,

> The main area where the extra A/D bit depth is helpful is in defining
> dark areas as image versus noise.

Possibly, but that depends on many other factors.  The CCD has noise, and
the analog front end has noise.  Only so many bits are actually useful, and
because of that, it's highly unlikely in this case that there really are 16
bits of noise free data.

> I have not compared the two, but a
> review I read claimed considerable benefit the extra bits the A/D
> converter provided.

I would have to see the review, but more than likely the "considerable
benefit" had nothing to do with the "extra bits".  You just can't add bits
and get better results, you have to have a better CCD and/or analog front
end that will allow these extra bits to be data that is not simply noise.

This is one of the issues that was discussed a while ago, is that
manufacturers can make a claim, like number of bits of the A/D, with
absolutely no substantiation as to whether it is meaningful or not...and
most customers have no idea that more does not mean better...and simply
believe that more bits must be better, heck, it's more.

> It is probably the principal weakness of the Dual
> Scan II, particularly the blue channel.  It is, in fact, that extra A/D
> bit conversion that should, in theory, help fix the problem you are
> claiming, that the dark areas are ill defined from the noise threshold.

The way to tell how many bits are useful (to at least a usable degree) is to
check the CCD spec and see what it has for a saturation point and a noise
floor, and calculate the dynamic range of that.

The Scan Dual III uses a three line (tri-linear) 2700 pixel/line sensor,
according to their spec.  Kodak doesn't make a sensor in that configuration,
so I'm not sure whose sensor they use.  If you find out what CCD these
scanners use, let me know, and I'll look up the specs.

Regards,

Austin

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