[filmscanners] RE: scanner dmax discussion

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

Hi Karl,

> But on A/D converters, the simplest way of doing it is very much along the
> lines of what you are describing - 1bit/step, but there is no inherent
> requirement to do it that way.

No, but since the CCD is basically logarithmic, as are our eyes, it works
out very nicely.

> As for the linearity of sensors, from the
> plots I've seen (and obviously Nikon, Canon et al aren't publishing their
> plots for competitive reasons), the devices are linear over a particular
> range, and while that is where they try to map the sensor into,
> at the ends
> you get significant non-linearity - noise on the low end and
> cascading/swamping/blooming on the high end.

Of course, the ends are not linear, but the answer is, don't use the ends!

> As for driving the scanners into different parts of the curve, I haven't
> seen any, and I've punted the question back to the A/D expert that claimed
> this to me.  Will have answer soon.

I actually have designed A/Ds (as well as scanners), so I'm a bit familiar
with how they work ;-)

> Your point about the minimum step increment being the noise function is
> correct.  The hard part is that I don't think the noise function is linear
> over the full dynamic range of the sensor so theoretically the step
> increments aren't linear either.

Actually, they really are quite linear...check out the linear sensor specs.

Regards,

Austin

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