crossover.

Knowledge stays in the fog of the past. Terminology that changes isn’t written in dictionaries of the future. Since those who use the term have left the field, those who were never in the field, those who weren’t trained or apprenticed, the story of original meaning, that of direct relationship, those people no longer enforce the meaning.

A crossover was a place, a part in a machine. Just as film machines at a place where the racks could be pulled out or pushed back into the line for additional processing. Pulling or processing from the soup.

Upon the diminution of film as primary image former, commercial labs ceased. Small labs can survive in small markets, erratic markets; dead cat bounces feed a starvation diet.

What (who) remains: people who could have worked in the big time but chose to take a hobby land approach. They learned from the magazines… those who don’t understand the process, those who look for why it won’t work; like a poorly trained law-clerk.

These people hear a word, and cling to the new found term, in hunger for a position of knowledge among others in the darkening room. They apply it widely. To almost every approximation, the word can be fit, it will be chived into use.

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Kodak filters for flashing

Filter	Color	Purpose
47 or 47B	blue	reduce yellow density in print
44A	cyan	reduce red density
58	green	reduce magenta density
32	magenta	reduce green density
25 or 29	red	reduce cyan density
32	yellow	reduce blue density in print

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Darkroom lab exposure have 3 stages of exposure possible. Think of them as being in relation to sequence of exposure to the unprocessed emulsion. An exposure before the image exposure, the image forming exposure, and the last being after the image has been exposed. These exposures combined are used to shape the curve shape of the finished image. This method was a key skill in pre-press labs. It was also used by commercial dye-transfer labs to a lesser degree, mainly because few labs had staff skilled in both commercial processes. Industry divided the knowledge into market segments, not the schools.

• bump [HL] exposure is short exposure without mask or screen
• flash [S] exposure is short exposure prior to copy, mask, or screen.. also a Raw, or Blank exposure
• main exposure controls location of midpoint as well as mid-tone scale. Developing is used to provide range of these scales.

In practice, this required multiple (2, or 3) light sources. One was the enlarger, the other(s) were lamps, likely modified to hold filters. In the typical shop, with such a setup, we routinely altered the crossover points of a characteristic curve.

It wasn’t a bad thing, it was routine, controlled, essential…

for more see