#3 provide a simplified interface

[Mojo Ze Jojo]

I think there would be an easy improvement: let's say you want to take a picture with a DSLR and you've made the measure, but you want to work out how long the exposure should be when using a ND400 filter, and the time is longer than 30sec.

To implement that:
- add a new "lense" (make the focal length very long to turn off automatically below extension correction)
- add a new "film" (or just make NRF xxx more "DSLR user friendly")