Thanks John. Good info.
george
John Petrush wrote:
> 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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