I realize that this thread is old and that Velvia 50 is no longer in production but I have to add my two cents worth.
First (this is a pet peeve of mine), you are not "over-exposing" film by changing the E.I. if the results are what you are after. I am amazed at how often the terms under-exposure and over-exposure are misused. For example, if you find that rating a film at it's box (ISO) speed produces dark transparencies, then you are under-exposing the film but you are not over-exposing it if you increase exposure by reducing the exposure index.
As for Velvia 50, from my experience, the vast majority (almost all!) large format photographers rate it at E.I. 40 or even E.I. 32. I do that as well. However, I have always found the 35mm version when rated at E.I. 50 to give me the best results in that format. But keep in mind that that can be due to different meters, shutters, etc.
I am surprised at how new film photographers are often afraid to rate film at other than the box speed. The box speed is simply a recommended starting point, that's all. The film manufacturer's will even tell you that. But some new photographers act as if the sky will fall if they change the E.I.
Also keep in mind that even if your meter and shutter is precisely calibrated and accurate, your metering changes everything. You could rate the film at it's ideal speed and then meter the scene improperly and get terrible results. This is fundamental.
What struck me about the original question is that David says that he found that he was getting better results at E.I. 40 but he isn't sure if he is ready to rate the film to E.I. 40! What's the issue? If you are getting better results at E.I. 40, switch to ISO 40. Your eyes are telling you that it's better but you were still (when you wrote this long ago) unsure whether to go with what you could see for yourself with your own eyes!!!??? David, your tests told you all you need to know. Why would you hesitate to rate the film at the speed that gives you the best results.
One more technical thing: When you rate a film at other than it's box speed, you are not chaning the ISO. In other words, if you rate Velvia 50 a E.I. (exposure index) 40, it is NOT ISO 40. ISO only refers to the box speed. When you use a speed other than box (ISO) speed, it is then referred to E.I. E.I. is the TRUE speed of the film for you with your equipment determined by testing.
There are almost no films that I rate at the ISO speed. Knoweledgeable photographers have no qualms at all and do not hesitate to test their film and rate it at whatever produce the best results. For example, I rate 100Tmax at E.I. 64, Tri-x sheet film at E.I. 200, Tri-X roll film at E.I. 250, etc. Those are NOT over-esposing the film. In fact, if I rated them at their ISO (box) speeds, I would be under-exposing them.
I just had to respond so that someone else who is unnecessarily agonizing over whether to rate a film at a speed other than the box speed will not heistate to do so. Again, the box speed is just a starting point and nothing more.
BTW, I have found that for me, with my lenses and meters, Velvia 100 performs best at about E.I. 125 but that's just me.
Last edited by ZoneIII; 02-09-2013 at 05:33 PM. Click to view previous post history.