I wish to thank you for this very clear posting. The subject line should read 'color' and I added that so our archive can have this excellent little dissertation.
Steve Shapiro, Carmel, CA
Printing negative color: If the print has too much red, add red to the
head. [etc. y etc. m etc. c et. al.]
----- Original Message ----- From: "Chris" <nimbo@xxxxxxxxxxxxxx>
To: "List for Photo/Imaging Educators - Professionals - Students"
<photoforum@xxxxxxxxxxxxxxxxxxxxxx>
Sent: Monday, January 17, 2005 4:34 PM
Subject: RE: Enlarger filter question
Sorry but Cyan is complementary to Red. Yellow to blue and Magenta to green.
There are three layers in photographic colour emulsion, as used in the C-41 process. The first is an ordinary silver-gelatine sensitive mainly to blue light. Then there is a yellow filter that removes the blue light. The next layer is orthochromic silver - gelatine that is sensitive to green light. Then there is a magenta filter which removes the green light. The remaining light, which is red illuminates a panchromatic layer which then contains the red image.
Within each layer are colour couplers which react with the colour developer as it is oxidised by the silver ions in the silver halide as they are reduced during development. Only silver halide grains which have had about three photons collisions and this have about three atoms of reduced silver go on to react in the presence of the developing agent.
The developer, as it is reduced becomes active and reacts with the colour developer to form a dye, which was not coloured before. The blue sensitive layer contains a colour coupler that forms a yellow dye (because it is the negative in film emulsions), The green sensitive layer has a coupler that forms a magenta dye and the red sensitive layer contains a coupler that forms a cyan dye.
When the film is fixed the sliver halide is removed and when it is bleached and fixed the developed silver is removed from the emulsion. There is then no silver in the film. Only the coloured layers remain.
I do not include any reference to the orange mask. But I think this has something to do with the initial blue sensitive layer and is to compensate for the varying sensitivity of each layer.
Fast film has bigger grains so forming a bigger target for any incident photons, making a strike more likely thus making fast film sensitive to low level of light.
Colour paper is the same without the mask, the silver crystals are very small in colour paper, making it less sensitive.. The old Agfa film had no mask.
I apologise to those who know more than me about this and to those who may feel their intelligence is being insulted.
It is of interest to me and, I hope to everyone else. I think I have posted something like this before.
My degree was in Chemistry and this was included in our lectures. I do remember something about the couplers and the colour developer, they are all organic.
We did discuss silverless light sensitive emulsions that only needed fixing by washing out the unreacted dye. The coupling occurred between two organic compounds when they were exposed to light. Another idea was that an organic dye would have a colourless form until exposed to light and then break into a coloured dye and another smaller compound which would be mopped up by another compound. Fixing is still required. Possibly this could be dome with ultra-violet light.
I understand some silverless emulsions have been used in the "spy satellites".
However digital photography has rendered further emulsion development obsolete.
Chris.
-----Original Message----- From: owner-photoforum@xxxxxxxxxxxxxxxxxxxxxx [mailto:owner-photoforum@xxxxxxxxxxxxxxxxxxxxxx] On Behalf Of SteveS Sent: 17 January 2005 01:13 To: List for Photo/Imaging Educators - Professionals - Students Subject: Re: Enlarger filter question
err: Sorry I wrote cyan, I meant the cyan sensitive crystols in the emulsion that are effected by YELLOW light.
S.
----- Original Message ----- From: "James B. Davis" <jbdavis@xxxxxxxxxxxxx>
To: "List for Photo/Imaging Educators - Professionals - Students"
<photoforum@xxxxxxxxxxxxxxxxxxxxxx>
Sent: Sunday, January 16, 2005 4:30 PM
Subject: Re: Enlarger filter question
On Sun, 16 Jan 2005 18:43:44 -0500, Walter Holt <locnleave@xxxxxxxxxxx> wrote/replied to:
You are preaching to the choir. I fully understand what you are saying as I started using VC paper in 1959 (DuPont Varigam I think). I was not the one asking about the blending or adding of VC filters, that was Kostas. Personally I think it would be a poor idea.
In my own darkroom I Use an Ilford Ilfospeed Multigrade enlarger head and control box. I just punch a button to change contrast. I am not unsure, but I do experiment experiment, and have fun.
I agree about not mixing filters - and not expecting too much. When you've increased contrast with a #4, that's it, forget trying to squeeze out more in any other way. Get a single grade if you need more contrast.
MG Filters are simply changing the colour. I have for years used colour head enlargerss with MG B&W and found the settings that worked best. #4 is a strong magenta filter while the softer filters are more light yellow. The multigrade papers have their limits and I've found it a waste of time trying to push those limits.
-- Jim Davis, Nature Photography, http://easternbeaver.com/ Motorcycle Relay Kits
