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Re: [OM] 80 macro

Subject: Re: [OM] 80 macro
From: fdayrit@xxxxxxx
Date: Sun, 28 Jun 1998 08:58:54 -0400

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

gary and john,

i just have to say that what you have been contributing are very valuable 
information that would take hours to days and lots and lots of mistakes to 
learn. these should definitely be archived where they are readily available!!

francis

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