[Leica] Re: Chemical HDRI explained

[lrzeitlin at optonline.net (Lawrence Zeitlin)]

Thanks to Bob Adler's explanation of HDRI, I sort of get it. HDRI is  
a method of compensating for extreme tonal range in color and B&W.

When I learned photography at the knee of Fox Talbot there was a  
chemical technique of doing much the same thing using either the  
carbro or the dye transfer process. Many old timers know about these  
methods but I suspect that most photographers too young to collect  
Social Security have no idea what I am talking about.

Both techniques are based on the fact that gelatin, when sensitized  
by dipping in a dichromate solution, becomes insoluble in water after  
being exposed to sunlight or UV light. Carbro is an additive color  
process while dye transfer is subtractive.

For color work in the dye transfer process, a set of three separation  
negatives were prepared and a sheet of gelatin matrix film exposed to  
each. When processed in warm water, the gelatin washed away from the  
areas that received lesser exposure, giving a differential gelatin  
thickness between the highlights and the shadow areas. Each matrix  
sheet was soaked in a dye solution, complimentary to the separation  
negative filter. The thicker gelatin areas absorbed more of the dye.  
Finally the dyes matrix sheets were carefully aligned, each in turn,  
over a sheet of transfer paper, firmly rolled into close contact, and  
the dyes would transfer to the contact sheet. The carbro process was  
essentially similar except that the gelatin on the transfer sheets  
had an imbedded pigment, either a colored pigment or carbon black.  
Immersion in warm water after exposure would wash away some of the  
pigment, the more exposed areas losing less. The individual sheets  
were then aligned and attached to an opaque white substrate.

These tedious and exacting methods produced the best color prints up  
through the end of the '50s. The couple of times I tried it, it took  
almost a day to make a single color print. Thank God for Kodacolor.

Anyway, this discussion of archaic color printing techniques has a  
relevance to HDRI methods. Instead of color separation negatives,  
photographers bracketed B&W exposures, exposing for the highlights  
and for shadows. Then prepared suitable carbro layers and laminated  
them to a white receiving sheet. It could be done with dye transfer  
with multiple  color separation negatives. I know that some nature  
photographers did this to deal with situations of extremely high  
tonal range. I was told that Ansel Adams dabbled in carbro printing  
before advocating the Zone system.

As for me, I usually used a compensating developer (D23), and trusted  
that Kodabromide had enough range to deal with my inadequacies. Then  
when I actually started getting paid for taking photographs (Boston  
Globe, '47 thru '52) my editor had no time for such nonsense. But now  
it seems much easier to do using layers in Photoshop or the  
Photomatrix system. Perhaps the term "matrix" in Photomatrix is a  
form of homage to the matrix sheets used in the old dye transfer  
process. Or perhaps not.

Larry Z