i.ortho.photo: DEM and photos resolution

Hi, I'm Luciano Caroti and I'm working on orthorectification using GRASS
4.1 on linux system. I have some questions:
        -Which is the optimal DEM resolutions to rectify 2m resolution photo?
        -What problems could I introduce into data using 100m DEM and 2m photos
?
        -Where could I find bibliographic references on this argument
(problems using DEM and photos having different resolution ) ?

Thanks in advance.

Luciano Caroti
Earth Sciences Department
Via G. La Pira n.4
50100 Florence
ITALY

Email: luciano@steno.geo.unifi.it