Add several sections

NOTES_ON_OPTICAL_PRINTER_TECHNIQUE.md

@@ -348,4 +348,259 @@ For this there is a simple rule.
 
 Optical printing "through the base" does not degrade image quality.
 
-The optical printer can also imitate the contact printer, if the original is inserted in the printer gate with emulsion toward the lens.
+The optical printer can also imitate the contact printer, if the original is inserted in the printer gate with emulsion toward the lens.
+
+
+## TIME
+
+The optical printer gives as absolute control over the flow or fits of time as the gods could have.
+But it's just a movie.
+
+Actually, two limitations on optical printer time manipulation are the grainy ground of film pictures and the mere 24 frames per second, as shown by these two examples.
+
+## FANCY FREEZE
+
+A single frame of the original printed repeatedly is a freeze frame. Unfortunately the running grain pattern of the original freezes with the picture.
+The picture has lost its ground. 
+This can be avoided if there are at least three frames without motion in the original, by alternately printing among the three.
+
+## FANCY SLOW
+
+To slow motion to 3/4 speed (as is required when original shot at 18fps is to be made into a 24fps print) it is usual to print every third frame twice.
+ABCDEFGHI... becomes ABCCDEFFGHII... . 
+The micro-freezes, just two frames long, coming every 1/6 second, are perceived through their rhythm.
+This can be avoided by randomizing the frames to be doubled while still choosing one frame from each three of the original.
+
+## DIFFUSERS
+
+For poorly designed condenser systems a diffuser will even the illumination over the frame.
+Other than for this, diffusers are located immediately behind the original film to alter image quality in several ways:
+1. reduce the appearance of scratches on the film base;
+2. soften the appearance of grain, and without reducing resolution *per se*, reduce sharpness;
+3. reduce the apparent oontrast of B&W originals, approximating the tonality in contact printing.
+
+Opal glass, a more extreme diffuser than groundglass, is more effective in each of the three ways.
+
+An opal glase oan reduce exposure by 4 or even more stops.
+An accidental or for;otten opal glass can devastate an exposure!
+
+## UV FILTER 
+
+A filter which absorbs the ultraviolet, such as Wratten 2B or 2E improves sharpness with almost all lenses.
+
+Also, printing color film onto color film, or B&W film onto color film, there is color reproduction advantage to using a UV filter.
+
+Some color rawstooks have such filtration built in, some don't.
+
+Even printing color film onto B&W film there is tonal advantage to using a UV filter.
+
+## IR FILTER
+
+A filter which absorbs (or reflects) the infrared keeps much of the light energy which would heat the original but not contribute to the photography, off the original.
+This filter must be located between the lamp and the original.
+
+Printing Kodachrome original onto color film, there is color reproduction advantage to using a filter which reflects the far red (past 670nm) and near infrared.
+Printing other color originals there is color reproduction disadvantage to using a filter (such as many heat filters) which remove the far red.
+
+## GREEN FILTER
+
+Printing B&W to B&W with a non-apochromatic lens, a green filter can improve sharpness.
+
+## FILTER LOCATION
+
+The spectral effect of a filter on the photography is the same wherever it is located between the lamp and the rawatock.
+The optical effect of a filter can't be good, so it ought to be located on the illumination side of the original rather than on the image-formation side.
+(There, flaws in filters are harmless. A color filter may even be perforated to reduce its effective saturation.)
+
+## EXPOSURE
+
+In optical printing as in original photography, the exposure is adjustable, and a necessary consideration.
+But there is a difference.
+The natural scene may exhibit an immense brightness range, from the brightest light sources (and secondary sources--reflections) to the darkest light sinks.
+The film original is limited in brightness range, between the clear of the base and the maximum density of the emulsion. 
+This could be an 11 atop range for some color reversal films, but only about 6 stops for a negative original.
+
+The exposure problem in original photography is to decide what portion of the immense brightness range to capture on the film.
+The exposure problem in optical printing is to decide how to capture on the print film the whole of the original image range.
+
+## EXPOSURE ADJUSTERS 
+
+**SHUTTER SPEED** - A variable speed motor or gearing can give a few stops of adjustment. Brevity is limited by inertia.
+A single-frame mechanism cannot be expected to complete a cycle in less than about .1 second.
+Slow running is unlimited, but emulsions misrespond to very long exposures, losing speed and gaining contrast.
+
+With the variable shutter the shutter speed (the time the light strikes a point in the frame) may be adjusted although the printer camera runs at just one speed.
+Exposure can be adjusted over several stops with the variable shutter. 
+Brevity is limited by the shutter mechanics which must give equal even exposures at the smallest shutter angles.
+
+To cut exposure by 1 stop using the variable shutter, halve the shutter angle.
+To cut another stop, halve it again.
+
+Using the variable shutter for exposure adjustment makes its use for fades or dissolves inconvenient.
+
+**LENS APERTURE** - This is a silly way to adjust exposure.
+Changing lens aperture changes picture sharpness.
+Except for fine expoeure adjustments (+/1 1/2 stop) the lens is best left at its sharpest opening.
+
+(For exposure testing and other dirty work, lens aperture is a handy exposure adjuster.)
+
+**LAMP VOLTAGE** - This is the classical way to adjust exposure for B&W printing.
+But it introduces color changes.
+Also, modern halogen lamps lose life at prolonged low voltages.
+Voltage adjustment is a practical means for fine exposure adjustment.
+
+Dropping the voltage 10% reduces the light about t stop while changing the color about `CCO5Y+CCO2M`.
+
+**POLARIZERS** - Two polarizers, one rotatable, is a cute way to adjust exposure.
+But sheet polarizers get hot and have short lives in the optical printer.
+Only very expensive ones can maintain color neutrality over a 10 stop adjustment range, and it is sad to fry them.
+
+**ND FILTERS** - These grey filters are the preferred way to adjust exposure.
+
+.30 of Neutral Density equals one stop.
+
+Neutral Density values add as filters are stacked.
+Thus an `ND.10` filter + an `ND.20` filter + an `ND.30` filter works like an `ND.60` filter, and this cuts the light 2 stops.
+Etc.
+
+.10 of Neutral Density equals 1/3 stop.
+
+A clear glass or film may be used as an `ND.035` filter for finer exposure adjustment.
+
+Long, graded ND filters allow continuous exposure adjustment.
+
+ND filters of higher value become hotter and have shorter lives.
+In a pack of ND filters, lower values should go toward the lamp.
+Past .60 it makes little difference.
+
+Wratten #96 gelatin ND filtere are expensive because of their optical quality, unnecessary where they are located in the optical printer.
+
+Lee theatrical filters #209, #210, and #211 are good for `ND.30`, `ND.60`, and `ND.90` and cost 1/100 as much as Wrattens.
+
+There are glass ND's, both absorptive and reflective, of great permanence.
+Also, developed B&W films, fine halftone screens, etc. can be used as ND filters.
+
+## SPECIFYING EXPOSURE 
+
+The many variables of exposure include:
+
+1. the type of original film and its pictorial qualities
+2. the type of print film (and if a lab stock its batch number)
+3. magnification
+4. lens aperture
+5. diffuser (if any)
+6. lamp voltage
+7. shutter angle
+8. non-ND filtration (as well as) 
+9. ND filters
+
+## FILM SPEED 
+
+ASA and related values are specialized to original picture taking and are not quite appropriate to optical printer applications.
+The values are informative for comparison of similar stocks.
+For many printing films ASA and related values are undefinable.
+
+The optical printer will have exposure standards unto itself, determined by testing.
+Once it is known how to beat expose, a certain original onto a certain print film, good estimates can be made for similar originals or similar print films.
+
+## RIGHT EXPOSURE 
+
+Working in reversal there is a temptation to want the optical print to match the original.
+Resist this temptation!
+You want the optical print that produces the best release print.
+
+(Even if the optical print must be intercut with the original, so that the two must produce matching release print, it doesn't follow that the two must match, and they shouldn't.)
+
+Starting from reversal camera original the best reversal optical print is typically a little (about `ND.20`) darker
+than the original.
+This avoids the print film's toe.
+The best reversal optical print of this will match it.
+And so on.
+
+Starting from negative camera Original the best interpositive print has some density in the highlights.
+The beat internegative is a little darker than the original negative.
+A further interpositive would best match the first one, etc.
+
+## GENERATIONS
+
+Gammas multiply. 
+For example, a gamma 1.5 original printed onto gamma 2 stock resembles a gamma 3 original.
+
+In many-generation pictorial optical printing a chain of gamma 1 steps results in unchanging picture contrast.
+7399 and CRI are gamma 1 color reversal stocks.
+7243 is a gamma 1 color negative stock.
+PXR and 7361 are gamma 1 B&W reversal stocks.
+7235 is a gamma 1 B&w negative stock.
+
+For B&W negative there is the option of alternating gammas above and below 1--7366 with gamma 1.4 and 7234 with gamma
+.7--and multiply out to 1.
+
+There are no available reversal stocks with Gamma less than 1.
+
+For color reversal ECO, until its disappearance in 1985, was a favorite gamma 1 camera stock and ECO--ECO--ECO--etc. was the classical printing scheme. 
+ECO--7399--7399--etc. was a similar, possibly better scheme.
+For each, only the release print would be on higher gamma stock.
+No present color reversal scheme has that advantage.
+Higher gamma original VNF--7399--7399--etc. is a printing scheme.
+For this, the release print too will be on 7399.
+Original Kodachrome follows the VNF scheme.
+
+7399 stock misbehaves with exposure times longer than about
+-1 second.
+
+For B&W reversal PXR--PXR--PXR--etc. is the classical printing scheme.
+PXR--7361--7361--etc. is a similar, slightly better
+scheme.
+For each, unless an opal diffuser is used the effective gamma is much greater than 1.
+
+For color negative ECN--7243--7243--etc. is the classical
+printing scheme.
+The alternating positive and negative pictures allow different manipulations.
+Optical printing from picture negative requires unusual cleanliness, to avoid white specks in the final image.
+A good strategy is to make the odd printing steps quick and simple, perhaps even contact printed.
+
+The shortcut scheme for color negative ECN--CRI--CRI--etc. comprises only picture negatives.
+
+For B&W negative there is a shortcut scheme BWN--7361--7361--etc.
+
+Tonal degradation sometimes confused with contrast increase may be due to misexposure, or to impossible exposure (as when the print film lacks the exposure range to handle the density range of the original).
+Then picture falls on toe or shoulder and is tonally compressed.
+
+Through generations graininess semi-adds.
+The grain of the original is in part added to the grain of the print stock.
+The print may thus look less grainy than the original, or more grainy, or just differently grainy.
+
+Through generations sharpness diminishes.
+The unsharpness of the original joins the unsharpness of the lens and the unsharpness of the print stock, in the print.
+Sharpness may be boosted, however, by boosting contrast.
+
+Optical printing with the best lenses onto relatively thick emulsion print films may be sharper than contact printing.
+Generally optical printing isn't as sharp.
+Picture degradation from generation to generation could be avoided by making the pictures very large, or by digitalizing them.
+But in this medium the original, intermediate, and final pictures are all of the same size, made in similar ways, of similar materials.
+Besides the practical economy, there is conceptual economy in this. 
+Intuitions transfer easily from one formally similar picture phase to another.
+Thus making the generations the same makes them different.
+This is the paradox, or the folly, of optical printing.
+
+## BELLOWS FORMULA
+
+Exposure way change with magnification.
+A "bellows formula" works for most printer lenses and most illumination systems.
+It prescribes...
+
+| magnification | compensation |
+|-----|-----|
+| `M = .13` | add `ND.50` |
+| `M = .18` | add `ND.46` |
+| `M = .25` | add `ND.41` |
+| `M = .35` | add `ND.34` |
+| `M = .50` | add `ND.25` |
+| `M = .71` | add `ND.14` |
+| `M = 1`   | normal |
+| `M = 1.4` | remove `ND.16` |
+| `M = 2`   | remove `ND.35` |
+| `M = 2.8` | remove `ND.56` |
+| `M = 4` 	| remove `ND.80` |
+| `M = 5.7` |remove `ND1.04` |
+| `M = 8`   |remove `ND1.31` |