Velvia at EI40 is for contrasy scenes, not the intended diffuse illumination that the film was designed for. At moderately short (<1 sec) EI40 will lighten shadows, but will also blow highlights easier. It is a risky thing to do and most risky with cameras using evaluative/multipattern/matrix meters because they do not provide implicit bias to elements (individual luminances) as a manual spot meter would. Ideally, meter the scene with a spot meter (Hi/MID/LO/AVG) at box speed (ISO50); nailing the exposure in the right light is very easy and produces beautiful, beautiful images (more difficult with 35mm, but it can be done, and done well), then even more startling when printed (by whatever means, either darkroom or digital). If people are getting blocked up shadows, grossly blown highlights and the like at EI32, EI40 or ISO50, the film is not being exposed correctly. None of this is a secret. All of the Velvia films (including the pasty 100F and bold 100) require diffuse lighting. Provia 100/100F is Velvia with a reduced primary palette spread and better shadow tolerance and highlight control. It is preferred in illumination that is not or could potentially throw Velvia off.