zenrhino said:
HCA w/1:20 selenium 2-3 minutes/these give you N+1
I'm unfamiliar with HCA and n+1 (other than as an algebraic concept). What do they mean?
HCA stands for Kodak's Hypo Clearing Agent which is a prepackaged clearing agent for films and papers. It is largely sodium sulfite.
N+1 is an abbreviation for an expansion development/toning of 1 stop. It is Zone System lingo which refers to an increase in contrast of 1 stop. This can be accomplished either through an exposure change or through development. In this case the reference is to increasing the density scale of the Polaroid instant negative by one stop through the action of the selenium toner. The highlight areas of the toned negative will become proportionately much denser than the shadow areas with this treatment.
The concept is similar in a way to "pushing" film of which you may have heard. To push a film one gives less exposure than normal (usually by rating the film speed higher by a stop) which could be thought of as N-1 exposure. When this is coupled with an N+1 development the contrast is adjusted higher and this compensates for the loss of contrast caused by the underexposure. This is a very simplified explanation of the concept but it is essentially what you do to "push" a film. "Pulling" is the opposite, combining a plus exposure with a minus development. The degree of "pushing" or "pulling" can also vary as in N+3, N-1/2, etc.
For Van Dyke Brownprinting this additional contrast will improve your results in most cases since the process requires a dense, high contrast negative for optimum results. That sort of negative is difficult to achieve just using the Polaroid film unless the subject lighting contrast can be carefully controlled.
