Added a few more sections and a graphics placeholder

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Matt McWilliams 2022-07-27 13:48:49 -04:00
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@ -777,3 +777,66 @@ CHART C
| 1.50 (5 stops)| 3.16| 5.7 | 4 | 7 |
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## IMAGE SUPERPOSITION
In an overall combination of two images, the two can infuse each other as lightness or as darkness, or they can be slapped onto each other.
There are three basic types of image superposition, named according to how they are made.
Pictures A & B combined by...
1. _Double exposure from positives._ The print film is eared twice, once from A's positive, once from B's positive.
2. _Double exposure from negatives._ The print film is exposed twice, once from A's negative, once from B's negative.
3. _Bipack._ Two films, either A's and B's positives, or else A's and B's negatives, are inserted together in the printer gate. The print film is exposed once, from this pair.
The print film is unspecified.
It is in the final positive print that the three types of combination are compared, and they look very different.
For B&W the differences can be described by how tones combine.
With (1), lightness dominates.
Where one tone combines with another tone the result is nearly the lighter of the two tones.
With (2), darkness dominates.
The result is nearly the darker of the two tones.
With (3), there is contrastification which complicates the tone combination.
If a bipack is examined ray (unprinted) wherever both images are clear the bipack is clear.
Wherever either image is black the bipack is at least that black.
Wherever both images are black the bipack is doubly black.
The bipack, which appears dark, has a tonal range doubling that of the
single images.
The bipack is unprintable in toto.
To abstract a picture from the unprintable bipack printing exposure is typically increased 1-4 stops.
With 4 stops increase, where clear and black coincide prints as a dark grey would—-not a clear domination of either lightness or darkness.
With 2 stops increase there is darkness domination.
## GAMMA & BIPACK
If the bipack is printed onto gamma $ material, to reduce the contrast to normal, it is a true tonal blender, without dominance, of the two images.
As the graphs below show, the gamma 1/2 bipack is the mean between the type (1) and type (2) double exposures.
## INCIDENTALLY
A type (3) of a type (1) and a type (2) is just a type (3) again.
Four idealized graphs summarize the three basic types of superposition and the gamma 1/2 bipack.
Example:
> 4 has density .75 and B has density 1.75 in one place. From the first graph, the double exposure from positives has density about 1.0 in that place.
**![Graphics depicting DOUBLE EXPOSURE FROM POS'S, BIPACK (1.0 COMPENS) DOUBLE EXPOSURE FROM NEGS and GAMMA 1/2 BIPACK](#)**
For a double exposure it doesn't matter which exposure is first, or what time separates the two.
In some multi-head optical printers, using a beam-splitter, the two exposures are simultaneous.
Either way, the two films can be independently adjusted for exposure, filtered, etc.
For a bipack it doesn't matter which film is in front.
Also the two may be optically instead of mechanically bipacked.
In some multi-head optical printers the films are in separate gates, ones projection becoming the other's illumination.
In a simple printer one film may be in the gate and the other in the camera, in front of the print film.
Any way, the two films ehare one exposure adjustment and filtration.
When films will be physically bipacked they should first be wiped with a lubricating film cleaner.
This is good practice for all optical printing when delicate originals receive heavy handling.