Re: Back-exposing on plastic

Katharine,

if you can obtain a full tonal scale by exposing from the back of the
substrate in a single exposure ie the first Marek print you put up for him
on your website.

This mean in a single exposure you can get full tonal scale this either from
the back or from the front but when the exposure is done from the front the
gum it seems can't hold on to the substrate be it paper or some transparency
(at least not completely). The reason I would be interested in exposing from
the front would be to obtain a better resolution and a bit more sharpness
and I assume the idea of doing this on a transparency is probably because it
is assume it can be easier to transfer image layer onto paper. From what I
understand, all this exercise is about obtaining in a single exposure a full
tonal scale and there is two approach that seem to be favored at this time.
The first is exposing a gum layer from the back of a transparency and this
result in your final print. The second is to expose a gum layer from the
front and transfer it on paper (before development). The benefit of doing
this in this manner would require a single exposure and less manipulations
but a person like you would have to learn an all new set of rules and
techniques and since you can probably make a gum print with one hand tied
behind your back as it is, all this may be more trouble then it's worth for
you.

In a previous message you seemed surprised that exposing through a piece of
glass resulted in very poor resolution and sharpness. The reason is similar
to what happens when you use a pinhole camera. Lets say your negative is a
clear circle of 1/8 inch surrounded in a high density black, your light
source (point source) is 12 inch from the negative and say your glass is 1/4
inch tick. Geometrically speaking you will have a ray or a line that will
start at this point source and go to one side of our circle and another ray
will go to the other side. Actually from the point source you will have a
cone of light starting at the light source and 12 inch away it will be 1/8
inch in diameter, a 1/4 inch farther it will be a bit bigger (0.1276 insted
of 0.125 (1/8)). Now, even if you use your usual photoflood lamp, this is
far from a theorical point source light and it will make our 1/8 inch circle
much bigger and blurrier, all this will be worst with diffuse light source.
As you get away from a theorical point source ligh, the cone of light
projected behind our circle increase in diameter to about 0.2109 inch which
is 1.6875 larger then 1/8 inch original circle.

Regards
Yves

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, April 18, 2006 12:36 AM
Subject: Re: Back-exposing on plastic

>
> On Apr 17, 2006, at 8:44 PM, etienne garbaux wrote:
>
> > Katharine wrote:
> >
> >
> >> But I don't understand where this transfer business is going, all of
> >> a sudden, because I thought that Marek had demonstrated that you can
> >> get a better tonal scale with gum by exposing from the back
> >>
> >
> > I don't think it's anything terribly obtuse, just that many folks
> > would
> > like a gum print on paper with a full (and continuous) tonal scale
> > (as can
> > apparently be gotten by exposing through the substrate), and not
> > everyone
> > (apparently) has given up on persuading an exposed and developed
> > gum layer
> > to release from its plastic substrate. I'm not sure I see any
> > advantage to
> > gum transfer over gelatin transfer (i.e., carbon transfer), but I'm
> > prepared to be amazed when someone does something with it that
> > can't be
> > done with carbon.
> >
>
>
> And so am I, obviously, but I think you've missed my point entirely.
> We seem to agree that it's by exposing through the substrate that one
> can get this full and continuous tonal scale that we've all been
> excited about, including myself (although I have not yet been able to
> replicate it in spite of working on it the entire day) but the
> transfer method Marek attempted today and the method Dave was
> recommmending requires, apparently, exposing from the top. Since
> one can get a better image exposing from the top directly onto paper
> than you can get exposing from the top onto plastic, my question is,
> what would be the advantage to transferring a less fully tonal image
> to paper than you could get by printing directly onto paper in the
> first place? That was my point.
>
> Katharine Thayer
Received on Tue Apr 18 04:33:25 2006