Reply #5 - April 12, 2007, 03:13:31 PM
Hi Leen!
I had to hit my head against the wall a few times to get my brain in gear- I think I damaged the wall but I now recall the work of Karl Schwazchild, the German physicist and astronomer who died shortly before the end of WWI.
His work in photography was mostly in the astronomical field where he did research into the optical density of photographic emulsions. If I am not mistaken, the scientific/mathematical formula was know as the Schwazchild exponent. It's been a long-long time since college days but I recall this had mostly to do with what we now refer to as reciprocity law failure in practical down-to-earth photography. I would imagine in Schwazchild day, with low speed materials and the earlier design of telescopic optics, some very lengthy exposures were need to obtain photographic images of distant stars. I think he was trying to establish the distance of certain stars my measuring their relative intensity on the photographic emulsions and the occurrence of reciprocity failure could have hampered theses attempts. I would guess his equation; i=f ( I.t(p) was a means of factoring in the failure.
In some of the older films this was an important factor to consider because many of the popular emulsions in bot color and black and white suffered from this kind of deficiency in both black and white and color. Many films manufactures recommended extended development times when exposing their black and white films with electronic flash because of the relatively short flash durations. Some of the early electronic flash units operated on extremely high voltage and comparatively low capacitance- this yielded flash durations of one one hundredth thousandth of a second. There were even specialized developers made for this specialized usage. Of. course there were many films that required longer than normal exposures to correct for long exposures where reciprocity failure would set in as low as 10 seconds. Reciprocity charts were issued in the instructions supplied with theses films. Enter color photography- now there were multi layered emulsions. During long exposures, not all of the layers would receive enough light at the same time and those dread color-crossovers would begin to occur. This was remedied by longer exposures and the addition of filters to help with correction the color response, or lack there of, in the various layers of the emulsion- HAPPY DAYS!
Still being a film user for a great deal of my work, I have found that the latest emulsions have a grater tolerance for longer exposures and there is very little or no reciprocity indications on many of the black and white, color negative and transparency materials I still use.
There are so many theories that are common to both film and digital photography but this one has me without any direct correlation or down pat answer. Since a lot of the old theory is based on chemical reactions of the development process, I would guess that theses do not directly apply to the electronics involved in digital imaging. Noise is so often compared to grain in cases of underexposure but again, I don't know if there is a direct correlation between the two. In film, sever lack of shadow detail will not only reveal grain but prevent a really good level of D-Max on the final print and will result in muddy color issues as well. In digital work the same kind of thing seems to occur but to my knowledge, this is caused by the electronic system in the camera "straining" to produce an image at graiter ISO settings when the light is below it's normal working level.
In videography, noise was a sever problem when the cameras used image-orthocon tubes and the related circuitry. Once sold-state devices took over- that is no longer a serious issue and the new digital video camera can work well even in extreme low light conditions. If the same technology applies to the latest digital cameras, I can not see a reciprocity-like problem occurring unless one is operating totally below the capabilities of the camera.
If there is any correlation between film and digital photography it might be a good Idea to realize that most high speed films work better in terms of graininess, D-Max, resolution and contrast when they are exposed at about 1/2 their published ISO rating. The Kodak Portra 800 is an exception to this rule, it is a true ISO 800.
Another interesting tip- I sometimes use reciprocity law failure to my advantage- another reason why I like film for certain assignments. I am often asked to photograph industrial or retail interiors which are lighted bu sodium-vapor and other lights which are lacking in the red portion of the visible spectrum. I use a daylight type color negative film such as Portra 400 and adjust the aperture at a very small f/stop which allows for a very long exposure. Theses films, when exposed for a couple of minutes develop a red bias which compensates for the lack of that color in the light sources. The resulting negative is absolutely normal and prints with ease. I have tried this with digital equipment and although the camera comes up with a white balance, it does not look as good as the film method. This might be some indication that the electronic sensor and it's circuitry are not subject to traditional theories pertaining to the reciprocity laws.
The next thing I will do is try to contact the technical folks at Nikon, Cannon and or Fugi and ask about obtaining information as to the nature of their systems as to longer than normal exposures and if there is a cut off point where the camera can not interpret long exposures beyond a particular range.
Interesting topic, to say the least! Regards, Ed Shapiro
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Ed Shapiro
The Hintonburg Studio
Suite 201 78 Hinton Avenue North
Ottawa, Ontario CANADA K1Y 0Z8
613-792-4837 Email:
edshapiro@rogers.com
) histograms I would have to conclude that yes, something like that would occur. As one channel or another gets to a higher value at a set exposure, it seems to me that a longer exposure would exaggerate the ratios.
