I agree with doolittle. For Velvia only around two years out of date, cold stored, I wouldn't vary from normal exposure at all.
You need to take comments about underexposing Velvia to Eg. boost saturation with a grain of salt. It depends very much on how you plan to view/use the processed transparencies as to whether you follow this advice or not.
If you prefer to view the slides via projection you can underexpose your film a little and enjoy the results when seen on a white screen. On the other hand, if you want to scan the frames, by rating the film higher than 50, you are only making life very much harder for yourself by underexposing a film that is already one of the most contrasty types available, and which many scanners will already struggle to extract good clean detail from what can (depending on the scene and your lighting, of course) be very dense shadow areas indeed.
If you have access to a high end scanner able to deliver clean shadow detail from Velvia 50 you might be willing to go this way anyway. I love projecting my 35mm Velvia frames (there is still nothing, repeat, nothing, in colour that looks as good as a well exposed, projected transparency in my view). But I also like to scan my films and Velvia can challenge any consumer level scanner even when it is perfectly exposed. For this reason, I tend to incident meter my transparency exposure settings, and quite often, when using my older cameras with full shutter speeds and half f stops, when faced with Eg. underexposing a quarter stop or over exposing by the same amount, I will usually go for slightly over, with a view to trying to keep the scanning process manageable.
It is very true that transparency films are unforgiving of overexposure. They have nothing like the latitude of black and white negative, or most colour negative films. It is sort of like digital imaging, in this respect (or, to be fair, like digital imaging used to be a few years ago). It is the highlights that blow out rapidly, which is precisely why incident light measurement works so well, as, when done correctly, this will prevent highlights blowing out just about every time.
Now having said all this, you can give Velvia a little extra light in most situations. As mentioned above, it can't take much over exposure but it will usually look OK up to about a half a stop "hot" and, if there are any shadow areas in the scene you would like to maximise your scan detail from, this can help the scanner pick this up. If you go more than about half a stop over, highlights will blow out rapidly and then it is all finished, there will be nothing to recover when you edit. Incident metering makes it much easier to judge where the highlights are. If you use a reflective metering process then, as you will have gathered, you will need to be fairly discriminating in what you take a reading off in terms of reflectivity and the light hitting it, as you can quite easily come to grief, otherwise.
These are general comments, as not all subjects are created equally, and, depending on your lighting conditions and subject, some scenes will naturally be much more challenging to image with this films dynamic range than others, and obviously Eg. subjects in open shade will be more manageable than, say, a pretty girl under an umbrella at the beach on a sunny day, or scenes with snow and shadows. Sometimes you just have to accept that you can't capture it all (or even close to it) and must make a creative choice according to subject, and whether the shadow or the highlight portions are more important to save. But generations of National Geographic photographers managed, in spite of these challenges, to produce superlative photographs of all manner of subjects with transparency films (Kodachrome, in many cases), so, do not be swayed by those who say you must shoot neg, or that it can't be done. History suggests otherwise. And when you get Velvia right, no other colour film still on the market today comes close to it and then it is all worth it.
Cheers,
Brett
