You are perfectly right.
In fact, the term *focal lenght* only applies to lenses (it's the distance
between lens rear nodal point and the focus point, when it focuses infinity
placed subjects); in a pinhole camera, it doesn' t exists.
Anyway, in order you can calculate the f-number of the pin hole camera, all
you have to do is to take two measurements: the pin hole diameter and the
pin hole-to-film distance.
To achieve wide angle or telephoto effects, all you have to do is moving
back and forth the pin hole plate. In any case, take into account two main
things:
- the image *circle* becomes smaller as you move the plate towards the
film and it can happens the image *circle* doesn' t fill totally the film
area;
- the f-number dramatically decreases as you move the plate away from the
film; in such conditions the light intensity inside de camera becomes
weaker and the difraction greatly increases.
Good shots