There is a conversion factor with the Digitals vs. film cameras. They don't see the subject the same. Thus a lens that would be standard for film at 50mm is actually equal to a 65mm lens on a digital camera. You need to convert the lens opening or apeture. For example, my favorite lens is a 50 macro so effectively with the digital its the same as a 65mm would be on a film camera. To get a wide angle view at 17mm with a digital is equal to 28mm film. Multiply 1.6 x the lens size to get the digital equal.

Sorry for the tech lingo. Please let me know if you need translation.