AREA NEWS
RECIPROCITY FAILURE Nick Williams (Area 20)
When you expose a film you use the law of reciprocity. This means that to obtain your exposures you expect a certain amount of light to fall on the film for a given time. If you want to, you can use twice the light for half the time or, half the light for twice the time.
If your exposure meter shows a correct exposure of 1/125 at F8, you can use instead 1/250 at F5.6 or 1/60 at F11. The reason for the peculiar aperture numbers is that they represent the focal length of the lens divided by the effective diameter of the aperture.
What we should be concerned with is the AREA of the hole which determines the amount of light that comes through.
If you double the diameter of the hole you multiply its area by 4. So, to double the area, you multiply its diameter by `root 2` which is approximately 1.4.
The law of the reciprocity holds very well over the normal range of exposure, but ceases to apply for exposures less than about 1/5000 sec. and for those over 1 second.
The following table gives a general guide to the compensation requires for the longer exposures.
COMPENSATION REQUIRED FOR LONGER EXPOSURES
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Indicated exposure (Secs.) 2. 3. 4. 8. 15. 30. 60 120.
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Multiply by : 1.25 1.33 1.5 1.5 1.62 2 2.33 3.75
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To give correct exposure 2. 5 4 6 13 30 70 180 450
==========================================================CIBACHROME CONTRAST CONTROL Terry Dennet (Area 20)
has obtained some details of a method for contrast of Cibachrome. An American company can supply a K-1 kit which will process at a temperature of 75 degrees F (24 degrees C). A second kit, the K-2 can be used at temperatures of 75 - 95 degrees F (24 to 39 degrees C) and can work with shorter paper exposure times.
The contrast control is achieved by separating the functions of the dye bleach and the silver bleach. No additional wash steps are required. The dye bleaching is not dependant on the mutual destruction of the silver image. It is strictly proportional to the silver image density. Because of this the dye bleach time may be varied to adjust the contrast of the print. Shorter bleach time will give less contrast while longer bleach time will give more contrast.
Both kits, at $13.75, are supplied as two small bottles of liquid, along with packets of powder chemicals. Apparently, shelf life is unlimited.
Kits are available from:- Dye Chrome Research Co. Inc., P.O. Box 969, Lake Placid, Florida 33852.
A.M. v E6 Oliver Barron (Area 20).
Do you use the Arnold Moorhouse formula or the E6 process, and which film do you process in the solutions? Is it the best combination?
Peter Guy, Sue and Ron Croad, Ron Knowles, Lawrence Edwards, Nick Williams, Glyn Willicombe and Tony Chuter all sent to Oliver barron a sample of their own home processed slides. The slides were mixed together to avoid recognition and posted to each participant for marking. This is a league table of the results as collected by Oliver:-
1st |
AM (Oct
79) |
Fuji |
E X H I B I T I O N
Some of our Northern Ireland members took the opportunity to run a CRC stand in conjunction with their local photographic Club. At the same event a shield for the best Club entries was presented in memory of the late Arthur McKee. The event was also held to commemorate 150 years of photography.
PHOTOGRAPHY THROUGH THE MICROSCOPE
If anyone is interested, I have a spare copy of the Kodak booklet "Photo- graphy through the Microscope" for free (+ postage) Write to me:- Reg Cornish
CINE FILM A. G. Sparkes (Area 22).Bill Crumplin, our stereo wide screen expert, is devoting more of his time to cine film. Instead of using commercial film he is another, like E. Davies of Denbigh, who reduces 35mm slide film to 9.5mm cine format, with appropriate perferations. Bill can do this himself, but in the case of Super 8mm film for another camera he is calling on the help of a fellow enthusiast with the right equipment for slitting and perforating.
Bill is pondering whether to go for Konica or Fuji Professional film. It`s a bit tricky processing long lengths of narrow cine film, even with the right tanks, so we await developments (!).
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Now is the time to sort out those summer slides, edit out incorrectly focussed or exposed shots, perhaps change the order and make some title slides.
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Plastic stacking boxes can be very useful, now only for storing equipment and processing solutions, but one of these makes a good water bath for Litre bottles of solutions and could be fitted with a low wattage heater. "Dialene" boxes come in three sizes and colours and are obtainable from hardware stores etc. at around £3.45 each.