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Matt McWilliams 2022-07-26 14:54:17 -04:00
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NOTES_ON_OPTICAL_PRINTER_TECHNIQUE.md
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@ -124,3 +124,72 @@ An 8mm original blown up to 16mm and projected will appear sharper than the same
If the blowup optics are good this is even true when the 1:1 printing is by contact.
Likewise for 16mm to 35mm.
(This is all due to the print film being in effect twice as sharp and half as grainy in a bigger frame.)
## PRINTER LENSES
A lens well-corrected for `M = 1` is less well-corrected for `M = 2` (or `M = 1/2`).
A lens well-corrected for `M = 2` is
less well-corrected for ` M = 4` (or `M = 1/2`).
Etc.
(Floating elements improve this.)
A lens well-corrected for `M = 1` for a larger format is lees than ideal for `M = 1` for a smaller format.
With such specialization (and expense) in optical printer optics what is the hope for the $50 50mm enlarger lens, optimized for `M = .1` and much too large a format?
Not bad, provided the sharpest aperture is found and heeded and focusing technique is good.
Also, for `M != 1` an asymmetrical lens should be mounted the right way, which is usually with its smaller glass facing the smaller image.
A very sharp cheap printer lens is the Canon Macrophoto 35mm f/2.8.
## OPTICAL ZOOM
Optical printers do not use zoom lenses, although they could.
An optical printer zoom is made by moving the camera and lens each frame, so as to vary magnification while holding focus.
It is a dolly shot!
A dolly shot is equivalent to a zoom for a flat subject.
Geometrically this zoom can be identical to a zoom had it been made in the original photography.
It can also be deviant, by tracking not to the center of the frame.
Pictorially the zoom gets grainy, showing that it was not made in the original photography.
Rather than focus at each frame, camera and lens positions can be precharted for, say, every 10th frame, and the other positions interpolated or computed.
On the J-K, counting the turns of the lead screw is a means of repeatable
positioning.
A follow-focus mechanism is a boon to optical zooms.
The rate and course of zooming is a factor of style, as it is in original cinematography.
## LENS APERTURE
For picture taking the printer lens should be at whichever aperture gives the sharpest pictures.
This is found in tests.
If a lens must be stopped down past f/8 to reach optimum it is a terrible printer lens.
## FOCUSING
Printer focusing procedure is different at different magnification.
At 1:1 the camera, not the lens, is moved for focusing.
Only at magnifications greater than about 1.4 is it better to move the lens for focusing.
Near the 1:1 setup lens motion has no focusing effect.
With the camera fixed in its 1:1 position lens motion: adjusts magnification between about M=.96 and M=1.04 (at f/5.6).
## FOCUSING APERTURE
With all but the best optical printer lenses either (1) focus at the taking aperture or (2) focus at a larger aperture and then shift focus by a pre-established distance before taking.
This "fudge-factor" is found in film tests.
## FOCUSING PRECISION
Especially when focusing stopped down, focus many times (perhaps 20) and set an average position.
## FOCUSING TARGET
Use whatever target is found easiest to focus on.
One caution: the fudge-factor is target dependent.
Very fine resolution targets may require different fudge factors than coarser targets do.
## DEPTH OF FIELD
At indicated f/5.6 there is already more than enough depth of field for a bipack, at 1:1.
Also it is unnecessary to refocus when adding the second film.
Likewise when a single film is reversed emulsion to base.
At larger apertures and at larger magnifications depth of field is less.