How did machines in the 80s and 90s print from a roll of developed negatives fed into it?
What makes those legacy machines different from a roll of negatives fed into today's machine for printing?
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My profession revolved around the printing machines you are asking about. I was technical manager for 7 giant photofinishing plants in the Southeastern United states. Each plant was sized to handle, on a daily basis the peak volume generated by the holiday season. My labs were sized to develop and print 20,000 rolls of color film a day. I worked for Eckerd Drugs, a chain of 2,000 drug stores. Photofinishing was big business in that era.
The rolls of film arrived at the lab in individual envelopes. The pre-printed envelopes were filled out by the customer with name, address and instructions as to number of prints per frame and what size prints wanted. The envelopes were opened, contents inspected, and sorted into bins by film type. Each roll, accompanied by its envelope was passed to a room in total darkness. Each bag and roll received an adhesive label with the same unique number. This twin check tied the roll to its envelope.
The twin checked rolls were spliced together making one long roll, hundreds of feet long. This long roll of spliced film was threaded onto a machine that transported the long roll into a series of vats filled with chemicals. The chemicals were maintained at a specific concentration and at a specific temperature. The duration time each roll remained in each chemical vat was precisely timed. The film exited the film as a roll of dry developed film.
The long roll of developed film was moved to the printing room. Dozens of automated printers and their operator worked under safelight conditions. These printers were loaded with a long roll of light sensitive photo paper. The printer transport mechanism advanced the long roll of film, frame by frame to a printing gate. Each frame is viewed by the printer operator and an exposure analyzer built into the printing machine. Based on experience, he/she can take no action or override the automated exposure assessment.
The printing machine’s shutter opened, an enlarged image of the film is projected on the light sensitive printing paper. The lamp-house contained automated colored filters that custom modify the color of the exposing light. Thus the exposure time and color of the exposing light applies corrections that counter camera exposure errors and lighting conditions at the time of film exposure. The rear of the print paper receives a printed number and letter code. The code corresponds to corrections applied to the color and timing of the exposure. They will be used if the finished print must be reprinted to improve its color balance.
The now exposed long roll of paper moves to an automated paper developing machine. The paper is transported through a series of chemical vats. The paper is developed and dried.
Next the developed paper, developed film, and envelope converge at an assembly station. An operator feeds paper and film into cutting machines. The film is cut into short strips and each is inserted into protective transparent sleeves. The operator inspects each print for off color. Prints that can be manually reprinted for improvement are culled. If no action is taken the film and prints are inserted into the respective envelopes. The finished package is returned to the drugstore.
Let me add, from about the mid 1950’s to the end of the 20th century, photofinishing machines evolved with higher exposure determination accuracy. Operators were able to see a computer generated image that pre-visualized the finished print. This allowed the operator to override and apply manual corrections. The goal always was a high quality product with little need to reprint a frame.
Today’s digital printing stands on the shoulders of a century of evolving exposure determination technology. It was my occupation, I loved every minute.
Prior to the introduction of scanning on the mass consumer level (late 1990s), all prints were made optically. By the time C-41 was mature (early 1980s), RA-4 was the standard for print chemistry -- the same process that's still used for both projection printing and laser prints in color.
The general operation was that negatives were processed in machines very similar to those still in use (roller transport or leader card type, for consumer labs, dip and dunk for professional, mostly), albeit without the integrated scanner now common. The uncut negative strips, once dry, were then fed into the printer where an human operator would handle setting (at least) the first frame position. Filtration was generally adjusted generically for film type (that is, all Kodacolor II would get the same filter pack, at least until there was a change in paper batch), without the rebalancing per frame that's common in today's scan-an-print systems.
Once the first frame was aligned, the machine would automatically print, advance by eight perforations, and print again. The most human intervention would be for an operator to watch a reflex viewer to ensure the framing stayed aligned (sometimes cameras will slip spacing -- overlap frames or insert extra space -- due to mechanical fault), and possibly push a button to accept framing.
Paper was supplied on a roll, the width of the print's smaller dimension. That is, for 4x6 ("four inch prints") from 35mm, the paper would be four inches wide. For labs that handled other formats (most of them, in the earlier part of the time frame) that same paper would be used for various 120 frame sizes (producing 3x4, 4x4, and 4x7 prints from the standard 120 frames), for 126 (4x4) and for 110 (4x6 again). The common sizes were three inch and four inch, often with a small price premium for the larger prints.
Prints came out of the printer in a strip and were cut apart (with a very slight trim to ensure the join was discarded) separately. A human normally had to select the starting point and format to be cut, and if a mistake was made you might wind up with a whole roll of prints in which a narrow strip of the next or previous frame was included and a narrow strip of the current frame cut off (and attached to the next or previous).
By contrast, in today's process, the negatives are scanned (usually at pretty low resolution, one or two megapixels) by the same machine that develops the film. Few local labs handle 120, even fewer 110, APS, or disc. The scan files are then transferred (usually via LAN) to the printing machine, which uses lasers in red, green, and blue to "write" the image directly to the RA-4 print paper, which is then developed and cut in much the same way it would have been in 1985 -- with the exception that the machine can now accurately cut the prints apart and stack them into roll batches without human intervention.
For these standard three inch and four inch print sizes, there's little if any difference in quality between the projection printing of 1985 and today's digital process, for correctly exposed film. Overexposed will print at corrected density values, usually almost indistinguishable up to a couple stops over, and underexposed will generally get green and grainy beyond one stop error, due to the automatic attempt to raise the overall brightness into the "normal" range (where projection prints made by machine would simply print very light, or very dark on earlier machines, and have slightly different correction artifacts on the last generation of optical commercial printers).