Originally Posted by
FrancisF
I have cobbled together a 20 x 24 with parts from Filip Harbart. I am using Ilford FP4 with an Apo Nikkor 1210 mm but I have not been getting consistent results because I have not worked out all the issues of BEF and RF. Things are looking underexposed.
If I am getting an incident meter reading of 20, the meter says I need a half second at f128.
If I have 6 feet of bellow (1829 mm) then my bellows extension factor should be:
(1892 squared) / (1210 squared) = factor of 2.28. This is just a little more than one stop adjustment to the exposure. This would increase the exposure from half a second to just a little over 1 second.
The FP4 film comes with a graph to adjust for long exposures. The graph seems to say that a “measured exposure” of 1 second should be adjusted to about 5 to 7 seconds. The graph is a little hard to ready – I am interpreting it.
Any corrections to my calculations or advice about how I should be thinking about this? Thanks!! Francis Fullam, Chicago, Illinois
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