Kennedy said:
[snip]
Is it better to adjust the levels (histogram) with the scanning
software or in Photoshop? I don't want to lose information when
scanning. I've read scantips.com, searched this group and googled. I
have alot of slides of artwork to scan. I don't want the fastest way,
I'd like to do it the best way.
Depends on the capability of the scanner and the modes that you are
using. As a minimum, no matter what scanner and mode, adjust the white
point (for slides) or the black point (for negatives) before scanning.
For scanners like the Nikon Coolscans and Minolta 5400, is adjusting the
white/black points before scanning done by the scanners' hw or sw?
That depends on which option you use to adjust the white point (in
slides) or the black point (in negatives). Of course there is no
advantage (other than the 16/15-bit per channel difference I mentioned
later in the post) of implementing the adjustment in software. If used
correctly, the auto-exposure will adjust the white point correctly in
hardware.
If it
is done by hw, are you referring to adjusting the light source' exposure
(the Coolscans' LED intensity and the 5400s' exposure time)?
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. Similarly, there is no
capability in any of the Nikon scanner series to adjust the CCD gain or
the gain of the analogue electronics between the CCD and the digital
convertor. Both of these misconceptions may well be attributed to
rather sloppy documentation in the Nikon applications and their manuals.
The *ONLY* adjustments available in the Nikon scanners concerning the
exposure are the exposure time or a post capture digital adjustment.
For
scanning slides, I don't believe it is possible to *only* adjust a light
source to get a good white point without losing ground at the black
point, and vice versa.
Lets consider slides first, purely for explanatory simplification -
negatives have the same issues (but with reversed polarities of course).
There is only one optimum exposure of the CCD to the slide. That is the
exposure which causes the lightest points in the slide (ie. the white
point) to just avoid reaching the saturation level of the CCD. Any
higher exposure will result in clipped whites. Any lower exposure will
result in more of the blacks being lost in the noise floor.
It is kind of like setting an exposure on a film
camera for either highlight details or shadow details, but not both.
That is true, hence the statement in the original post "as a minimum".
If you wish to go beyond that minimum then you must indulge in merging
multiple scans with different exposures.
However, you should note that you cannot establish a "black point" on
slides by exposure adjustment on the scanner as you can with film. All
the exposure adjustment can change is the level of black which can be
discerned from the noise floor, however black will actually appear much
lighter in the resulting data as a consequence of the exposure. This is
because the minimum signal output by the CCD is determined by dark
current, not the lowest level of illumination. Thus increasing the
exposure not only has the obvious effect of increasing the optical
density at which the noise floor of the system resides, but of
increasing the amount of dark current accumulated. This results in the
black point actually increasing, or becoming lighter in the CCD output.
Contrast this with increased exposure on film, as your analogy, where
the darker details can be detected by the film but true black remains
black on the film itself.
In short, you cannot directly establish a black point in hardware when
scanning slides - the black point is a consequence of the scanner's
dynamic range and the setting of the white point. The black point
setting can only be implemented after the scan when the signal is
digitised and usually by software. Some older scanners, eg. Nikon
Ls-1000, LS-20 (& LS-30 pre-firmware upgrade), did implement this in
hardware, but it was still implemented after the CCD signal had been
digitised.
By permitting the whites to saturate you can scan higher film densities,
but the black point in the image (before any software black point
setting) will be lighter as a consequence of the CCD dark current.
Consequently the dynamic range of the scanner is reduced.
The scanner's sw or PS can of course do both. But if adjusting
white/black points is done by the scanners' sw, what is the advantage of
doing it before scanning over doing it in PS after scanning?
The main advantage, at least with the Nikon scanner range, is that
NikonScan is a true 16-bit per channel package, whilst Photoshop is
(unless things have changed in CS) only a 15-bit per channel package.
I think that my above comments and questions about w/b point adjustment
are applicable to gamma adjustment also, aside from the 16 vs 15 bit per
channel difference.
Gamma is *always* implemented on the digital data (ie. effectively in
software even if through a hardware lookup table as in the older Nikon
scanners) with CCD based scanners. This is *the* major performance
limitation of CCD scanners compared to photomultiplier based drum
scanners, where the majority of the gamma is implemented in the PM tube
itself and only fine adjustments of that curve require implementing
later.
CCDs are inherently linear devices and the non-uniformity in dark
current and response on each cell means that the gamma cannot readily be
implemented in the analogue signal path. The consequence of this is
that the difference between a 15-bit and 16-bit gamma compensation for
both MAC 1.8 and PC 2.2 standards is quite visible in the final result.
For example, in a non-toe-limited 2.2 gamma compensation, the lowest
non-zero level which can be represented on a 16-bit system transposes to
1.66 on the 8-bit display range, whilst for 15-bit processing it is
2.27. Now that might not sound like much, but it is a systematic 37%
additional quantisation error on all pixels near black (over and above
the 50% increase in quantisation error across the board) and that is
what makes it visible.
