I was shooting velvia on a 6 x 7 rollex back at small apertures of f32 or f45 ca lling for 1 sec exposure. I rated the film at 50 which probably would do better at 40. Nevertheless, exposures were dark at least 1 to 1.5 stops under. The mete ring seemed right and contrast not a problem. Could velvia have a light failure problem at one second requiring additional compensation exposure?
In my experience, Velvia needs double exposure time somewhere between 30s and 1min. There is no way Velvia would need it at 1s. Your metering was most likely off.
It could be a defective meter, but your experience reminds me also of when I sta rted using that film. I soon realized that with Velvia it's better not to use incident light metering, especi ally if the scene is not evenly lit, but rather use reflective spot measurements. Determine a middle gray value (gras s, etc.) or better: use a simplified zone system measurement method, and rate the film as you say at 40 AS A. Reciprocity failure for Velvia is from 4 sec. +1/3 and 5 CC M, from 16 sec. +2/3 and 10 CC M.
In other terms, in seconds, measurement-compensated exposure time: 1-1, 4-5, 6-8 , 8-12, 10-16, 15-24, 20-40, 30-64, 45-100, (60-150, 90-250). Don't bother with magenta filtering unle ss you have no mean to apply color corrections in post production.
Kerry Thalman wrote a nice article in View Camera magazine one or two issues ago that touched on this a little. You might check it out.
I'd check this out: http://people.smu.edu/rmonagha/mf/reciprocity.html
He's got a reciprocity failure chart for Velvia, among other helpful items.
Also remember to allow for bellows factor for anything inside infinity focus. It is often amazing how many quater stops you loose for even a little bit further than the lens focal length. I recommend the spreadsheet printout that you can tailor to your lens that is linked to off The Large Format Photography Homepage. http://www.ai.sri.com/~luong/photography/lf
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