Emulsive have written a fantastic article covering slide film, Velvia specifically, and how to account for reciprocity when shooting in low-light conditions or using ND filters.
This is especially timely for me, as I’ve been wondering whether it’s worth trying longer exposures with Velvia, my favourite colour film. As it’s only ISO 50, it will need some working out with regards to reciprocity failure (over-compensating for a diminishing level of effectiveness when you expose the film for longer than a few seconds), and this guide gives some handy figures. At over £10 a roll, I don’t want to lose too many shots!
It also features some beautiful Velvia sunsets, and a few shots with the (considerably cheaper) Agfa Precisa too. I’ve had good results from Precisa, but it does also have a limited latitude (the range of tones a film can show). My shoot at Keston Ponds had some lovely rich tones in darker conditions, more washed-out colours in bright sunlight, and a lot of ruined, dark frames.