Talking of film, when I was teaching photography I used to keep my pupils spell bound by making a print using my own home made paper. I used art paper and covered it with a mixture of gelatin and silver nitrate solution which I then soaked in sodium chloride solution. The paper was dried in a drying oven and exposed using an enlarger with a normal b/w film that we took during the previous portrait session. The paper was tested for speed using a bit cut off exposed in strips at different times as per usual. I also used to make film and print developer, stop bath and fixer including high speed fixer that used ammonium thiosulphate. We made the baths for bleaching the film too. I got the formulation for the chemistry from the Ilford Book of Photography. We obtained quite acceptable results too with our pupils taking qualifications in photography. We used the Zenit camera made in the USSR as it was a cheap SLR of acceptable quality. I recall measuring the shutter speed using a beam of light, a photocell and a timer. Then the school closed and I was made redundant until I found work as an ITC technician. Chris -----Original Message----- From: owner-photoforum@xxxxxxxxxxxxxxxxxx [mailto:owner-photoforum@xxxxxxxxxxxxxxxxxx] On Behalf Of karl shah-jenner Sent: 21 November 2008 06:47 To: List for Photo/Imaging Educators - Professionals - Students Subject: Re: Long digital exposures? reciprocity failure of film is caused by the film needing X number of photons of light to strike the silver halide grain to knock of an electron - and then the gelatin has to absorb the h+ ion.. if not enough photons belt that halide because their coming in too slow, or the exposure is to quick then it satys firmly afixed. nice analogy with the stove/gunk too :) Now doping films such that the gelatin is a lot more receptive to gathering the stray ions makes film behave a lot better and reciprocity is overcome to a large extent - and it also becomes more sensitive. Agfas experimental formate doped films never made it to market whcih is a huge shame because the traditional 100 ISO film would have had better recirpocity AND an effective speed of 1000 ISO. (likewise 3200 would have become 32,000 iso :)
