Re: filmscanners: =?iso-8859-1?Q?Re=3A=20autolevels=20was=20re=3A=20filmscann...

[RogerMillerPhoto@aol.com]

Tony, I have no quarrel with your statements (I almost never do) except with 
your two-phased processing approach to restore detail in a specular 
highlight.  It won't work with a true specular highlight which is what the 
original question was about, but only with a diffuse highlight.  A specular 
highlight, by definition, is the reflection of a light source by a 
"mirror-like” surface.  Specular highlights include such things as sunlight 
reflected off a chrome bumper on a car, sunlight reflected off a ripple in a 
lake, or a studio light reflected off a model’s wire rim glasses.  Assuming 
the light source doesn’t have any inherent detail of its own, then its 
mirrored reflection won’t either.  So there’s no detail to recover and it’s 
OK to let the specular highlight go solid white on the final print, which 
means that it’s not a problem if the white point is set so as to clip 
specular highlights.  In addition, the specular highlight is bound to be so 
much brighter than the film can record for the exposure being used that it 
will have been “burned into” the film and would have lost any detail if it 
had any to start with.  For transparency film, the specular highlights will 
be the transparent film base. A diffuse highlight, on the other hand, will be 
almost as transparent as the film base, but not quite, and will be capable of 
holding detail.  So the diffuse highlight is the thing to use when setting 
the white point. The white point should be set so that a bright diffuse 
highlight, which will have detail, is almost, but not quite, clipped.  That 
will allow it to be as bright as possible in the final print, but not so 
bright as to lose its detail.  (If you don’t have a bright diffuse highlight, 
but only a darker grayer one, then setting the white point becomes a bit of a 
trial and error process as another post has indicated.)  I hope this isn't 
beating a dead horse (my apologies to the horse) and that someone on the list 
will find it useful.  The bottom line is that specular highlights ought to be 
ignored when setting the white point since its their nature to "blow out" and 
go completely white.  Instead, concentrate on setting the white point for the 
defuse highlight.  Similarly, the black point should be set so as to capture 
detail in a shadow area that actually has detail, but black areas without 
detail should be clipped to oblivion.  It's the stuff in between that gives 
me the most difficulty.

Boy I'm long winded, aren't I.  And out of sorts, too.  I just got my new 
SprintScan 120 today and it has a "high density" SCSI connector that isn't 
compatible with the connectors on my SCSI card or the SS4000 to which I want 
to daisy chain it.  I need to buy a new cable or an adapter, but can't do it 
until next week due to deadlines I have to meet.  It's like to the scanners 
were designed by two different Polaroid groups.  Argggg.

In a message dated 7/26/2001 10:01:04 AM Pacific Daylight Time, 
TonySleep@halftone.co.uk writes:


> > If so, why is a point of sun reflection in a photograph not a good
> > point to use for the white point?  Because it's not representative of the
> > majority of the image?
>
> Yes, it is massively out of step with the brightness range of the rest of 
> the image. If you set it as a white point, the rest of the histogram will 
> likely be scrunched up to the left - better to clip the specular highlight 
> and adjust for the rest. Or selectively mask and separately adjust two 
> layers then recombine, if you want to keep some detail in the specular 
> highlights.
>
> Regards 
>
> Tony Sleep