[filmscanners] RE: Density vs Dynamic range

["Austin Franklin" <darkroom@ix.netcom.com>]

> I would appreciate your input on that.

Hi Robert,

What they are doing is measuring a test target that IS calibrated, like the
calibration target for a densitometer.  It's calibrated to an absolute
scale, therefore, all measurements specified are relative to each other
between scanners/manufacturers.  This is absolutely the correct way to
characterize a scanner's performance for dynamic range.  That is how I've
done it in the past when I've had to characterize an imaging system.

I've solved my confusion about their dMin and dMax definitions with respect
to the equation dMax - dMin, and I was mistaken, that they do have them
correct.  The thing that confused me, is they are measuring density values,
which are inversely proportional to the CCD output voltage...and we were
just discussing CCD output and A/D input...doh!

I thought they may have them reversed...since an SNR of 1, means the signal
is equal to the noise and that should be on the right side of the
equation...BUT...when the SNR is 1, that is dMax (darkest reading), since
that's where the voltage out of the CCD is the lowest, as far as output from
the CCD goes...but it is the highest density value.  Also, the saturation
point of the CCD (clipping) is when you are measuring dMin (lightest
reading), and that's where the CCD voltage is the highest...but the density
value the lowest!  Doh again!

But, all in all, it doesn't matter with respect to the "discussion", from
what I can tell, and by the example I used in the last post, they are saying
the exact same thing as I was saying.

Regards,

Austin

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