In article <49msal$mlo@news.uit.no>, paalj@ibg.uit.no says...
>This ISO 40 business of Velvia makes me wonder.... I've tried Velvia at >ISO 50 in four different cameras and all slides came out overexposed. >Now I expose it at ISO 64, but ISO 80 is probably more on target. Note >that all other slide films I use, Kodachrome 25, 64, 200 and >Echtachrome elite, all get perfectly exposed at their rated ISO values >in all my cameras. Can anyone explain whats happening?

No. I consider K25, K64 the speed reference films, and most other films rate correctly when the camera meters are calibrated to those two.
Exceptions I have found: Ektachrome 100 HC (125 instead of 100); K200
(close, but very slightly dense at 200); Elite and Ektachrome 400 HC (320 instead of 400); Fujichrome 400 (320 instead of 400); Velvia
(40 instead of 50 -- BUT, if the subject is primarily green, I will
start boosting ASA, with 50-64 being closer to correct [Velvia is
DEFINITELY a quirky film!). Velvia is very contrasty (and maybe odd in color response) - maybe something in your metering technique, which would normally work well with other films, catches one or more of Velvia's quirks?
Hope this helps.