Re: emulsion side down and histograms

"Kennedy McEwen" <rkm@xxxxxxxxxxxxxxxxxx> wrote in message
news:J8BMmpBaWQwCFwwl@xxxxxxxxxxxxxxxxxxxxxxx
> In article <d9pc06$nkj$1$8302bc10@xxxxxxxxxxxxxxxx>, John
> <warthog@xxxxxxxxxxxxxxxxxx> writes
> >"Kennedy McEwen" <rkm@xxxxxxxxxxxxxxxxxx> wrote in message
> >KtzcCpEsFgvCFwo$@kennedym.demon.co.uk">news:KtzcCpEsFgvCFwo$@kennedym.demon.co.uk...
> >> In article <42BD3785.31E59460@xxxxxxx>, JJackson382@xxxxxxx writes
> >> >
> >> >
> >> >Kennedy McEwen wrote:
> >> >>
> >> >> In article <230620051942375501%lrcullen@xxxxxxxxxxxxxxxxxxxx>, Linda
> >> >> <lrcullen@xxxxxxxxxxxxxxxxxxxx> writes
> >> >
> >>
> >> In *both* cases the adjustment is made by changing the exposure time.
> >> There is *NO* capability in any of the Nikon scanner series to change
> >> the LED intensity, only the exposure time.
> >
> >Are you sure?
>
> Yes, absolutely sure. Linearly changing the intensity of an LED is not
> trivial, requiring electronic control of a precision far beyond what is
> capable of building into a consumer scanner.
>
> >Try the following test: In NikonScan, set analogue gain as
> >follows:
> >Red = +2; Green = Blue = -2.
> >
> >Load a slide with a white mount, so that you can see the reflected light
> >from the scanner LEDs when the scanner head approaches the front of the
> >scanner. Now do a scan (NOT a preview - the preview scan doesn't seem to
be
> >affected) and watch the colour of the reflected light from the slide
mount.
> >If your scanner is like mine (Coolscan 4000) then you will see red light,
> >proving surely that the analogue gain controls at least alter the LED
> >intensities.
> >
> No, it doesn't prove that at all. This same "proof" that the LED
> intensity changes has been raised in this group several times so I
> assume it must either be coming from a common origin or be a rather
> trivial misconception that many people make without thinking through the
> detail of what they are looking at.
>
> So, for the umpteenth time on this forum, let me explain what you are
> seeing with your own eyes to you!
>
> The scanner scans a typical 35nn frame in around 40seconds with about
> 6000 scan lines. During each scan line, the scanner cycles the LEDs
> through red, green and blue (as well as infrared, if ICE is enabled).
> That means that the scanner conducts about 18,000 typical exposures in
> 40 seconds so, neglecting the readout time from the CCD and the time to
> mechanically step from one row to the next, that makes the exposure to
> each colour around 2mS long. Even if the exposure is increased by +2
> under analogue gain control, it is, at most 8mS long. If the exposure
> is reduced by -2 in the analogue gain then the LED pulse is only 0.5mS
> long.
>
> Now, the persistence of vision in your eye means that the minimum that
> you perceive any pulse of light to last is around 50mS-200mS depending
> on the ambient illumination level. Darker ambients produce longer
> persistence. This is the entire principle that movie films and
> television rely on - that you cannot distinguish short bright pulses of
> light from long dark pulses once the pulse time is short - they both
> look the same. A short pulse lasting twice as long as normal with the
> same intensity will appear to your eye as a single pulse which is twice
> as intense as normal.
>
> So, look at the figures and *think* about what you are seeing. You
> cannot discern the intensity or the width of the pulses which are
> shorter than typically 100mS long, yet the scanner is pulsing its LEDs
> at 0.5-8mS according to the analogue gain control. So the longer pulses
> simply *appear* to your eye to be brighter - but they are simply the
> same LED intensity for a longer time.
>
> Hence, when you increase the analogue gain of the scanner in one colour,
> the apparent colour of the light leaking from the front aperture at the
> end of the scan appears to change colour - just as the colour of a pixel
> on a plasma TV changes colour depending on the amount of time each cell
> is illuminated: there is no amplitude control on the illumination of the
> scanner LED or the plasma pixel.
>
> Now, there is *one* way to view the illumination time of the LEDs in the
> scanner and confirm that time control, and not intensity control, is
> used. View the scanner aperture (and thus the light from the LEDs)
> through a spinning mirror or polygon. This will cause the LED pulses to
> spread out over the angle that the polygon turns during the exposure,
> the size of that angle is proportional to the pulse time and the image
> intensity directly viewable. You will see red, greed and blue images of
> the scan aperture at different positions and smeared over different
> angular subtenses. Of course, the polygon needs to be synchronised with
> the LED cycling rate, but with a variable speed control it is possible
> to do that over a typical scan period.
>
> A simpler test is just to measure the scan time with different analogue
> gain settings and confirm that it takes longer to make a scan at +2 than
> at -2. The readout time of the CCD and the stepper motor time, both
> ignored above, means that this is not a simple proportional relationship
> with analogue gain, but it is possible to determine the "dead" time
> between exposures and the constant of proportionality by a simple linear
> curve fit.
>
> >I have been confused about this for some time,
>
> Clearly. I hope the explanation has cleared it up for you.

Yes it has - thanks! Sorry if you've said all this before - I haven't seen
it.
>
> > in fact since I read the
> >Vuescan Help file - Ed Hamrick makes a similar statement to you along the
> >lines of "Analogue Gain is Nikon's term for CCD exposure time". It is a
> >little odd, therefore, that Vuescan's analogue gain control has exactly
the
> >same effect!
>
> Why would you think it should be different? The software you scan with
> has not affected the way your eyes have evolved!

Ignore my above statement - it was bourne of my misunderstanding of the
scanner's working.

--
John
Replace 'nospam' with 'todnet' when replying.


.

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