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Re: Epson or HP for alt-phot ... ? (some OT)

> Although I
>  greatly appreciate the work accomplished by Dan Burkholder, Dave Fokos,
>  and others, I am attempting a different approach.  Not better, maybe not
>  worse, but different.

Hi Jeff,

If we want the same straightforward scanning to printing (that is, matching 
our negatives to Pt/Pd or whatever process w/o personal preference in 
deliberate change of tonality) but we use different approaches to achieve 
them, then it doesn't matter whether we do it with Burkholder's, Foko's, 
Mathias', Soemarko's or whatever method, we will get mathematical 
equivalences. None is better or worse than another.

>  Some of the differences follow:
>  No monitor calibration is needed.

For direct scan to output calibration without online image editing, monitor 
calibration is not needed. Strictly speaking, the monitor does not even need 
to exists (if we can remember what key to punch in).

However, if one wants to do some kind of image editing (burning, dodging, 
etc), then it is good to have the monitor calibrated to at least approximate 
the look of the final print. One can try to do all numerical approach 
(associating a tone with an rgb value, but the final effect is difficult to 
judge until printed).

>  Levels define the maximum range of black and white (0% and 100%, not 5%
>  and 95%).

There are problems with using full range. First there is physical problem, 
but it depends on your output media. If it can support separation from 95% to 
100%, then you can of course use it; but there is also problem with tone 
mapping. Remember rgb value correspond to density in a log fashion, so 
"linear" steps would be rgb = 1, 2, 4, 8, 16, etc. Now you can see that if 
you increase the level by 1, you don't change the highlight much but you 
completely change the shadow by one step! What is not apparent to many is 
that when you are using say 5% to 95% (and calibrate accordingly), you are 
actually using a different gamma so that the distribution of highlights and 
shadows are more even linearly in rgb. This is getting too much to discuss in 
this list, so I will stop here. Those interested can check Photoshop or other 
digital scanning/printing newsgroups or mailing list.

Also, you want to avoid using 100% (or rgb=0), that is because the gamma 
function (the slider that you can drag in the "level" dialog box) does not 
change rgb=0. Gamma is a nice function that changes tonality "exactly" like 
zone compression / expansion. But if rgb=0 is used and the value cannot be 
changed, you might get ugly spots on shadow.

>  Curves are determined sort of backwards by adjusting the scanned
>  information to give a known, calibrated print value (not using a
>  calibration image).

Yes, what you are doing is closed-loop calibration which is basically the 
same thing as working with tone-reproduction curves except that you use rgb 
values rather than density values. For example, original subjects --> 
original subjects, densities of negatives --> rgb values of digital camera, 
densities of photographic prints --> densities of digital print). The concept 
is exactly the same.

As in traditional pictorial (pictorial as opposed to scientific in this 
context) photography, the desired reproduction curve is not really linear. So 
whether we do it traditionally or digitally, we will need to find the desired 
curve. Because of human vision, the desired mapping basically has something 
to do with curves for the tones of shadows to midtone and curve for the tones 
from midtone to highlight. In digital, of course we can go finer and work on 
1/4 tone, midtone, and 3/4 tone, and that should be well enough for our 
purposes.

It is nice to go through all this once as it is challenging, satisfying, and 
educating; but as I posted about 2 weeks ago, after you are done, you might 
get something that Photoshop is already giving you although finding that in 
itself is not bad.  :-)


Dave Soemarko

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