Velvia: ISO 50 or 40 ... or How About 100 Speed ?
Fuji Velvia slide film is great stuff. It is widely used by professionals for its combination of fine grain and color saturation. What isn't widely agreed on though is what ISO speed to rate Velvia at.
First, some background: manufacturers have very stringent industry standards for testing film speed involving gray scale density measurements under controlled conditions resulting from known exposures. Testing for Velvia is no exception, so technically speaking, it must be ISO 50.
Velvia, however, is a very high contrast and high color saturation film, rather unlike most other reversal films. And these facts get lost in that single ISO 50 rating. Because of its high contrast, you may find that it can be difficult to maintain detail in shadow areas when shooting Velvia at its nominal rating. Setting your camera at ISO 40 allows the image to overexpose about a third of a stop and can help offset this problem. After testing it both ways, I decided that I preferred it at 40 but you'll need to do your own tests as our shooting styles may not be the same. Simply take a shot at ISO 50, then change your camera to ISO 40, re-meter and shoot a second image. Do this with a roll of typical subjects keeping track of which were shot at which speed. When you're done, just have the lab to develop it normally as you don't want to actually push or pull the film. When you get the slides back, spread them out on a light table and see what you think. Since the contrast issue can also affect very bright subjects, I've also heard of people who shoot high key shots at ISO 64. Personally, I've never found this to be an issue although I do watch the highlights to avoid blowing them out. If you find yourself overexposing some bright shots with Velvia, you may want to test this option as well. When you get done, there's nothing that says you can't decide to shoot some subjects at 40 or even 32, some at the nominal speed of 50, and others at 64 or perhaps even 80. If you get the results you like, then set your camera to the speed that worked for you.
As a comparison, Kodak's E100 VS is also a highly saturated film and somewhat high contrast (although not as much as Velvia). How it differs from Velvia though is that it's main problem area is at the highlight end rather than the shadow end. It's not uncommon to get burned out highlights with E100 VS. Right now, I still shoot it at ISO 100 but I'm considering doing some tests and possibly rerating it as 125 depending on the results.
Fuji is expected to very shortly release a 100 speed version of Velvia. No word yet whether it will have this same contrast issue or not.
Update 4/22/2003 - It's official. Velvia 100F has been announced, and it sounds great! Fuji's website has all the details here.
Update 7/04/2003 - Velvia 100F is now out and available. Early reports sound pretty good although I have not tried it myself. Problem is, now that I'm shooting mainly digital, I have no idea whether I should run out and buy Velvia 100F or not. What a dilema....
Update 2/10/2005 - Fuji has just announced another new version of Velvia 100. The press release is subtitled "Successor to Velvia 50 Offers Higher Speed and Finer Grain," so it would seem this time they mean it. Reports are that this is a further refinement of the current 100F version. No word on how this compares to the other Velvia 100 that has been availble only in Japan.