<dan.c.quinn@[EMAIL PROTECTED]
> wrote in message
news:1193174524.449215.97660@[EMAIL PROTECTED]
> RE:On Oct 21, 11:35 am, piterengel
> <pslavi...@[EMAIL PROTECTED]
> wrote:
>>
>> Hi, becaise it no more possible to find lith film I need
>> to try with a
>> qute common film to obtain very high contrasted pictures.
>> I have Efke
>> KB 25 and Rollei PAN 25 at home. Can anybody suggest a
>> developer
>> to have extremely contrasted subjects? > Thanks all - P.
>>
>
> A variety of lith films are available here in the USA.
> Lith films
> are slow orthochromatic films which are processed in well
> lighted
> darkrooms; the same level of lighting used to process
> Graded Paper.
> Any film I'd think would lith process. Panchromatic film
> processing
> would be done in complete darkness. So, slow ortho films
> are used.
> As for developer a very low sulfite carbonate plus
> hydroquinone
> mix will likely work well; does for paper. Dan
>
Lithographic films were made in all spectral
sensitivies, pan films were used for making half-tone plates
for three or four color letterpress reproduction. The main
property of lith films which differentiates them for
pictorial films is their contrast. The contrast of a film
depends on certain features of its emulsion, mostly the
range of sensitivities of the silver halide particals which
make it up. Pictorial film has wide range of halide
sensitivity, lith film a very narrow range. So, for a lith
film the difference between an exposure which results in
full development and one which does not expose a partical
enough to develop it is very small. The result is that the
image is essentially either full density or none.
Lith film was used originall for photo-mechanical
reproduction of line or half-tone work, the latter using a
screen. The original half-tone process used a variation of
the wet-plate Collodion process because the senstive
coating (not an emulsion) could be very high contrast and
was very thin which maintained the sharpness of the dots or
lines. Later various types of lithographic dry plates were
used including some with the half-tone screen built in.
There were also a variety of "commercial" films, often
blue sensitive but also ortho or panchromatic, meant for
very high contrast but lower than the lith films. All of
this stuff has been replaced by digital methods for
photo-mechanical purposes but lith film remains because it
is used in a number of alternative photographic processes
and for special effects such as masking.
The lith developers using Paraformaldehide must be low
sulfite because sulfite interfers with the infectious
development property. Infectious development is the
accelerating development of silver halide particals near a
developing one. This produces a sort of chain reaction which
results in very high density and very high contrast.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
|
