Gary Reese <PCACala@xxxxxxx> wrote:
"Contrast control in slide copying can be achieved by pre-flashing the special
slide copying films. That gives a low level fogging exposure to the shadows
in your original slide. The fog exposure plus slide exposure will help keep
the shadows from blacking out. The technique is covered in books about slide
copying. It is akin to fogging at Zone 1 or Zone II in books about the Zone
System. Slide copies like the Bowens Illumitron (if I recall the name
correctly) have circuitry to pre-flash the emulsion. "
Pre-flash as a contrast control procedure works exactly as described. The
Bowens slide duplicating machine combines a pre-flash with a dichroic filter
head for precise color balancing. This delicate color balance shifts from
batch to batch of film, with local voltage fluctuations and the age of the
bulb. Its a fussy business. Another more radical technique is to make a
contrast mask of the original slide, sandwich the two together, and duplicate
the stack. The mask is an under-exposed and under-developed B&W contact
"print" of the slide. The approach serves to bring the highlights to the
shadows; the opposite approach of pre-flashing. This method is useful for
extreme contrast scenes and is also applicable to creating direct positive
prints (Cibas). Both methods compress the total tonal range.
The problem with high reproduction ratios and small lens apertures is
diffraction of light through the diaphram. The light rays no longer all
converge at a point, making precise focus impossible. Too small an aperture
does nothing for DoF and kills the sharpness of the details over the whole
image. Its better to reduce the reproduction ratio for a given focal length
lens and gain DoF that way, or use a shorter focal length lens, increasing the
reproduction ratio and gaining DoF because less extension is required. That's
why a range of macro lenses, from 20mm to 135mm, is needed (at least that's my
personal rationalization <g>). However, it doesn't take too long before DoF is
hopelessly small no matter what you do, so you do your best to arrange your
subject angle to as thin a plane as possible anyway.
John P
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