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 :) but it was not to be. As a historical aside, other bases (albumin etc) were lousy at grabbing stray ions so development had to be done shortly after exposure of the neg. Gelatin was pretty good stuff :) - home doping is still undertaken by film shooters of astro stuff. at least, it *can* be done.. Digital doesn't have this issue, but it has it's own problems as has been mentioned re noise. every electron might well be gathered and added to the whole.. but then so will every stray electron generated by the heat of the sensor. the astro technique of adding or overlaying shorter exposures and doing a few darkfield subtractions gets around this for astro work, but it's not *that* good for more conventional photos. software, whether in camera or post camera like Neatimage CAN do a fine job of eliminating noise, unless the sensor gets swamped. but it can also get things wrong. In-camera algorithms are probably cranked up a bit when exposures approaching the maximum a camera can shoot at are set - and these maximum exposure times are probably carefull set by the manufacturer to ensure a photographer doesn't exceed what they have determined as being acceptable limits. sadly, these can't be exceeded if the camera in question has a maximum bulb time for those interested in digital astro work, the canon D20A is discussed here: http://www.dl-digital.com/astrophoto/20Da-FirstLight.html
