Austin wrote:
>> I don't think anyone commented on my suggestion that a 14 bit A/D still
>> gives more detail in the middle part of the range of values (where colour
>> neg film generally is) precisely because the noise is lowest there?
>I understand what you are saying, but I don't believe that's what is done,
>or that it really helps in the way I believe you mean. The A/D will convert
>the same delta V over the entire capture range.
I understand that the delta V is the same. The point is that the comparison
between the signal voltage and delta V is much more favourable in the middle
to upper areas of the capture range.
> Typically, the image data only falls in part of the range of the CCD,
> and should be more in the middle, not the ends.
Well, that's part of my point. Assuming CCD output voltage rises with light
intensity, a really dark section of a slide will produce the lowest voltage,
which may get lost in the thermal noise of the CCD. The signal to noise
ratio should improve as the voltage increases, so that the middle to higher
light intensities should produce much more accurate samples than really
dark areas. In the case of a neg, the darkest areas (actually the brightest
areas of the original scene) are still nowhere as dark as a slide. So the
majority of the actual image information of a neg, or the midtones of a
slide should be in the best part of the CCD response and have a really good
signal to noise ratio - so 14 bit accuracy is actually *useful* for these
areas but less so for the more dense areas of the film.
In the case of a neg where you want to expand the subtle range of tonal
shifts in the neg, surely the more bits the merrier?
Rob
Rob Geraghty [EMAIL PROTECTED]
http://wordweb.com
